Can we revise the schedule??

I think you will be speculating if you revise the schedule without knowing the Time impact of th change. The proper think to do is assess the impact properly since it seem to be a major one and then reflect that on your cost and schedule baselines following the approval of the change and its impacts by the CCB.

I'm not familiar with all the acronyms you're using but I think I understand the question. Your front end engineering design (i.e. pre-planning) used a set of assumptions that differed from your detailed engineering plan developed later, which resulted in a large variance which I'm assuming is your SPI.

You might develop a notional schedule and identify the critical path(s), but I would absolutely not commit the entire schedule without knowing the impacts to cost and flow. If you are on critical path, you might need to commit certain tasks, as a risk mitigation plan to the entire program schedule in order to make the schedule feasible. We refer to this as "partial-commitment"

Without knowing the impacts, committing to an entirely new schedule involving unknowns can be taking on massive risk. Yes I have encountered this situation many times, and very often we find that the first re-plan reveals new problems that we have to go work before we have a workable plan.