Hello Dhruv,
There is no standard process to update new status profile and respective statuses on old contracts.
SAP also recommends not change/replace status profile once system has gone live.
You have below options:
1. Continue to use the old documents as they are, until their life cycle is complete.
OR
2. Create new transactions with data from old transactions and new status profile. Close down old transactions
OR
3. Update all possible link tables for old transactions to access the new status profile and respective statuses
I hope this helps.
Thanks,
Kapil