The part-time option allows students to determine their study course duration, in agreement with the University, considering to their time availability and commitment needs. The part-time application is irrevocable, once passed to part time you can not go back to regular time. With the transition to part time the number and the sequence of exams provided by your course system don't change, it only changes the time within the credits can be obtained.
Students who are enrolled in old-school courses, DM 270/04 or 509/09 students who have completed the maximum period for obtaining their degree, students enrolled in distance courses in agreement with the Consorzio Nettuno as well as courses in teledidattica Unitelma Sapienza, can't benefit from the part-time option. 

You should switch to part-time when you enrol in a course, if you already know you have little time to study or if you are a little later than the regular duration of your course. Remember that the new regulations (both the 509 and the 270) state that  the degree should be achieved within a certain time (the regular course years plus its double).
It is also advisable to switch to part time if you are enrolled over the expected duration of the course: in fact, as stated in Article 29 of the Student Regulations, students enrolling out of the expected duration of the course from the third year, incur an increase of 50% of the amount taxes.
If in Academic Year 2019-20 a student enrolls in the third year out of expected duration of the course and decides to switch to part-time, he will have to pay the first part of the fees (by November 4, 2019) and apply for part-time (by December 20, 2019).
The first and second part of the fees will be paid already increased by 50%, then, once the part-time process is completed, the third part will be updated with the 10% reduction expected, and it will be applied to the ordinary amount and the surcharge will be deducted paid with the first and second parts of the fees. If the total amount to pay will be less than the amount already paid with the first and second parts of the fees, the student will credit the excess amount for subsequent payments, or he can request a refund from the Student Office he belongs to.

The application can be submitted on Infostud from 24 hours after payment of the first instalment and within the deadline laid down in the Regulations, following the procedures indicated at https://www.uniroma1.it/en/pagina/infostud-part-time.

After having completed the Infostud procedure, the application is automatically forwarded to your faculty. You will receive notification of the submission at your e-mail address. Once the application is presented, you have 7 days maximum to cancel it.

Faculties approve applications by January 31 and the system automatically alerts students via e-mail of the result. You can only apply for part-time once. Once you have been accepted into the part-time regime, you cannot return to full-time.

No, you would follow the course you choose. 

The application must indicate the number of credits that you want to obtain each year: from a minimum of 18 to a maximum of 45. The faculty will examine the applications and based on the number of credits agreed with the student will be re-determined the duration of the studies. Example: a student out of prescribed time for a Bachelor's degree (180 credits) who has acquired 60 credits and asks for 20 credits a year: how many years will your course of study last? Simple: 180/20 = 9, the course will be 9 years. The student, having already made 60 credits, will be enrolled in the fourth year in course therefore the effective duration will be 6 years. Read More

Yes, you can switch to the part-time option before and after having paid the first part of the fees. Remember that the deadline for payments for students enrolled in years following the first is Nov. 4, 2019, and that the form must be printed from Infostud. 

The results will be sent via email by Jan 31, 2019 at the address stated on Infostud. Each part-time application is evaluated, the faculty can only change the number of credits the student will have to get every year and the year to which the student will be enrolled. 
For further information on the evaluation or submission of the application please contact the part-time spokespersons
Check the list 

If, for example, the student has agreed to acquire 20 credits and only has 18, he can recover in the following years. If the student succeeds in obtaining the title within the agreed period, nothing happens, if instead it goes out of prescrobed time compared to the agreed time, starting from the first year out of time of part-time course he will pay the taxes in the ordinary amount due, increased by 50 %. Remember, however, that those who opt for the part-time regime must achieve the title within a duration equal to twice the agreed time. If the agreed duration is 6 years, you must graduate in maximum 12 years.

If, on the other hand, more credits are obtained than expected, the "risk" is to graduate before the time established with the part-time: it can be done, but the fees for the agreed period must be paid.

Yes. The tax reduction is progressive: the first year of part-time is paid 10% less than the amount foreseen, the following years there is a gradual reduction, up to 40% less than taxes. The tax reduction is accounted for on the third part of the fees, which will be determined by the difference between the amount due to an ordinary reduced amount of 10%, minus the amount paid for the first and second parts of the fees. The career and the bulletin for the payment of the third part of the fees will be updated towards the end of February and in any case in time for the payment of the third part of taxes.
Attention: the tax reduction for students who take part-time is valid only for the agreed period; if it goes out of course compared to the agreed duration, it is necessary to pay the taxes in the ordinary amount, increased by 50%.

In case of transition to another course or in case of change of order, the student who wants to keep the part-time study regime will have to re-apply for the part-time starting from the month of November of the year following the one in which he changed course or order. The tax reduction will be applied from the beginning, with the progression described in the previous point 10. The course year and the study plan will be determined by the competent faculty bodies, on the basis of career reconstruction.

You will have to submit the application from the month of November of the year following the one in which you changed university.