When I do that, it opens again and continues scanning. Still 15 weeks behind
Clarify: does it
say it's "rescanning", or "synchronising"? It can't continue scanning if you swapped the wallet files. It can only resume a rescan if it's still using the same wallet file. And if it's synchronising, a rescan is not useful anyway.
Odd. I thought a rescan was often necessary after restoring a backup. I'll have to search through threads and see if I can find evidence of that.
The wallet.dat file stores the height of the most recent block, and blocks beyond that (if any) are automatically scanned for new transactions on startup. A full rescan is normally unnecessary and pointless.
Because simply closing Bitcoin client and restarting it (with the name changed to wallet.dat), does not give me my balance.
That's damned unusual. Are you sure you renamed the file correctly? How large is the wallet.dat file?
Sure, your bitcoin will Show in your wallet, when you have imported your wallet.dat right and when it has downloaded/scanned the last block.
The last part is incorrect. It is not necessary to even have any blocks at all for your balance to be displayed (though it will be displayed as "unconfirmed" in that case).