My plan is to move everything from old wallet in offline Bitcoin Core to Ledger, then once all that's confirmed on the blockchain I guess the old wallet becomes redundant?
Keep your old wallet for 2 reasons: Forkcoins and to have a plausible story for taxes.
I was not aware of the change address mechanism, so please help me clarify:
- small (test amounts) outgoing transactions from my old wallet will use a change address from my keypool
Correct. Large amounts use a change address too, unless you send everything.
- Bitcoin Core is offline so won't know about change returned
It knows, but won't know when it's confirmed.
- I presume that once I sort my Ledger Wallet, the blockchain will link all change transactions to it?
Correct.
I will make multiple backups as I go, but I suppose my anxiety is that because my old wallet is offline and I am try to move all to Ledger, that my change will disappear into the aether (as I don't fully understand change address mechanisms)...
Your 1000 key keypool is good, but it's probably created by your current wallet, not your old wallet. That means all your existing backups are
useless for change, so create a new backup of your wallet.dat (or more than one) before sending anything.