Merging two Data Mart tables

hi,

I have the following scenario

A bank has recently choosen a new trading platform (a new software). While merging the software with the existing one they had their own way of desingning the datamart/tables . Current Data Mart has only transaction level data and not Aggregated one..Since we are going to create a new ScHEMA on the top of this current datamart we are having some challenges.

We are trying to merge two tables

a. PRE-MERGING the table name is ACCOUNT_BOOK_KEEPING_PRE_MERGE which has just 45 columns
b. POST-MERGING another table is created called ACCT_BKPG_POST_MERGE which has 100 columns

So now two tables exist.
We are trying to do the design in such a way that we will have only one single table which maintains this Account Book keeping for pre and post merger.

My question:
What are all the design considerations we should think of during merging those tables to a single table