The technical document Nika linked is a good start.
Unfortunately if you are a large implementation with tens of thousands of projects and plans, this will be incredibly painful.
We recently had to do this due to fiscal period changes. In summary you need to "unwind" your use of the fiscal periods, then put it all back in the new periods. There's a lot of logic to get right and a lot to go wrong, so I'd recommend to engage with an experienced consultant. (i.e. a partner or CA services)
Please get in touch if you want to know more