Dear Amol
proposal:
1.) create new characteristic with the "wished" data type
2.) link the new characteristic to the same class and bring/transport it to prod system.
3.) create e.g an expert rule set (or e.g. a userexit based function module) to read the data (proposal: only try it for active VATs) do some mapping (old characteristic new characteristic) and write it back to database
4.) remove the characteristic from the class so that nobody can maintain it any more
Other options are:
Don't go on with a rule set but write a "one and only" ABAP report to do so and execute it as background job (on weekend; no user should be editing the data here).
To help "business" you could define specs in "packages" so that is know on whcih spexc maintenance would be allowed and which not.
Afterwards you need to may be execuet some minor tasks as may be create new report symbol, check e.g. any WWI template if the old one is still used etc.
PLease check othee class/chaarcteristic based options as well. SAP is delivering a high number of "mass data operation there (most related to deletion but it could make sense to infest some time regarding further options)
Other options as well could be:
start with completely new value assignment type and class; do the "same" mapping as above and then remove old VAT from property tree
In any case there is a big but ! As you may be now: EH&S does write changes in CDHDR, CDPOS; therefore if you go on as proposed you can get "strange" results looking back in history; it is not a good idea to manipulate CDHDR and CDPOS.
A further idea could be: go on as proposed but change the old characteristic to "display" only (this is possible if i remember correct); Now in the class you would have old and new value at the same time and may be this approach fits better the business demand
C.B.