How can I sync my devices to new PC without erasing files?

Ok, I've seen similar posts, so I'm guessing that this may be a stupid question, but I want to ask again according to my specific situation.

I have an iPhone 4S, an iPad 2, and an iPad Air. I had the first two synced on my laptop (which died last year)..but I manually manage my music, movies, etc. I bought a new desktop PC and I have no problem connecting to it, transferring purchases, syncing app updates, manually adding music and movies, etc. Life as usual. However, if I go to the device window and select one of the tabs like Movies and then click 'Sync Movies', then it gives me a warning saying that my device is synced with another computer and everything must be deleted to sync with this new PC and I'm give the option to 'Erase and sync'.

Well, I have no problem whatsoever by just manually managing files as it is - it ONLY seems to affect the auto sync for movies and music. And I have hundreds of apps and songs on my devices....plus, I always lose saved data on games and such whenever I restore from backup, so I don't want to have to go through the trouble of erasing and resyncing with the new computer.... But if I ever wanted to turn on the auto sync option for movies or music, is there a way to avoid erasing everything?

Also, my new iPad Air has never been synced with my old laptop. It's only been synced to the new one. But it gives me the exact same message (I guess because when I first bought it and did initial setup, I chose to load the backup from iCloud using the backup from my iPad 2). But this device itself has NEVER actually been synced with the old PC, so why should I be getting that message?

So, I'm sure my question is exactly the same as hundreds of others, so I apologize for redundancy, but any info would be appreciated.