Hii Aman..

Provisional allotment letter is the seat allotment letter which is provided to a student online by the board after allotment of a seat in the college.

The yes upgradation form is available online for a student so the student is not needed to report at the reporting centres for that form. Although once a student have to report at the reporting centre when the board will direct the student to do so.

The time for reporting at the reporting centres will be provided online on the website of WBUT. Last year the timing was from 10 a.m. to 4:00 p.m. but I would better recommend you to confirm it from the official website of WBUT.

After first round of allotment of college if in the second round you are given any seat of more preferred college then your seat will be upgraded otherwise the last allotted college seat will remain yours. Although if in the second round any college is allotted to you then you cannot refer to the college seat that you were allotted in last round. 

Hope it helps. 

Best of luck for your future.