Does bitcoin qt need to synch with the network before it will notice the change has been received
It won't know about the change transaction until it has processed the block containing that transaction. Your change transaction is in a recent block, your bitcoin software is still catching up earlier blocks and hasn't retrieved that recent block yet.
allow the big transfer to go ahead ... It is still rebuilding the blockchain
This seems like a misunderstanding on your part. If the block containing the "transfer" has 6 confirmations, it has already "gone ahead". It is absolutely irrelevant whether your wallet ever catches up or not. In this scenario wallets only ever do two things
send your transaction out to a few Bitcoin nodes it knows about hoping they will pass it on and that eventually a miner will notice it and include it in a newly mined block.
keep listening for newly published blocks and reading them to see if they contain any mention of any of the set of addresses the wallet thinks you are interested in (ones it generated for you).
Wallets do not "allow the transfer to go ahead". As far as the rest of the world is concerned, once your wallet sent out the candidate transaction, your wallet's role in the process ended.
Wallets generally try to dissuade you from using money associated with transaction outputs the wallet has either not yet seen in a mined block or for which it has not seen at least 6 subsequent blocks built on the block containing the transaction.
Footnote:
For simplicity, and to avoid causing confusion, I have omitted possibilities like RBF, double-spending attempts, etc. I am assuming the normal simple process used in most ordinary regular boring plain transactions.
