When you use a later image you can indeed just continue posting data if you configure the local setup to use the same node id’s as before, emonhub.conf may need editing as your emonTx and emonTH payloads may not match what your older devices transmit. You will also need to be sure you have 868 and not 433 selected there too.
Be careful with the “update emonBase” and “update emonpi” buttons in emoncms admin. It could stop your data if you use the wrong one and may involve a manually reinstalling the FW if you have a certain vintage rfm2pi v2.
see Update EmonPi Button or Update EmonBase Button?
The apikeys for your emoncms.org account should never change, you will need to use the same apikey you were using before.