Expense allcoation done can be changed from GL basis to fixed%?

Hi,

We have performed full procedure which are in expense allocation to allocate the cost on items. The method we were using was GL expense allocation basis and completed that successfully. now I want expense allocation to be done on fixed% allocation. if we replace current allocation  definition from GL to fixed would that replace it or we need to first reverse it and than we re perform the full steps.

Regards

Sanjay