In your case replacement should work fine, you would want to just grab the original utm_campaign value as a variable and add the variant letter, then you would replace the utmCampaign field in google analytics with this variable.
This could all be achieved with GTM by:
- Storing the utm_campaign as a variable (Query parameter).
I would however question whether this is truly the best way to go about getting the data you are after, why not have two campaigns each with their landing page set to the appropriate variant?
Anyway, I presume you have your reasons, but if you go down the data manipulation route, be sure to test thoroughly before messing around with your campaign attributions.