Why would you want to not do it? Id personally not cheap out - penny wise pound foolish. If youw ant to run an experiment, youll find out how well the battery lasts without doing so.
I believe there are non-BMW/mini scan tools that will code a battery replacement. Not sure if they will re-code a new size or type, but both are parameters to be coded in if battery type is changed.
Assuming that the same type of battery is just replaced, that is a simpler coding.