We are several versions behind (currently 2.4.7) and we want to upgrade, but we thought it would be good to replicate our environement locally and then to do the upgrade as well as customize the templates then. That way when we upgrade the live version the downtime will be minimal and all of our templates will be ready.
Is there an issue with doing this?
No not at all. The only thing you want to be careful of is to not setup your crons (or delete the scheduled tasks if using the windows installer) so that your duplicated database doesn’t start pulling in real email from your mailboxes.
Of course if you’re using live data you always need to be careful of anything that may email the customer as well (automation rules, making test notes, etc) but lots of customers do this and is a good idea for getting your template changes in place.