Can you swap Legacy tenders with Legacy locomotives without harm?

If your Lionel engines and tenders have not been modified with connections between them then its Ok.

You won't damage anything, worst case you won't get sounds or maybe only some. An early Legacy engine may not be able to drive later legacy tenders. Later engines can drive early tenders. Some of the earliest Legacy engines used the TMCC drawbars with the IR devices under the drawbar so the IRs won't line up. These are pretty rare though.

Pete

fwiw, the PE tender has the IR drawbar. The chuffing sounds are actually very robust from the tender...a bit different sounding to my ears from the 2014 PE Berkshire.

Obviously, you won't damage anything with just the IR link.   As stated, most combo's will work, but some won't. 

Specifically, I had the Legacy LionMaster T1 Duplex with modular electronics and RailSounds 6 and the newer Legacy C&O Mallet with the RCMC and the RailSounds Lite board.  The Mallet had no problem with the T1 Duplex RS6 Tender, but the LM T1 Duplex locomotive and the Mallet tender were a different matter.  I couldn't get any sound, but the serial link worked as I could fire the coupler and turn the reverse light on and off.  Obviously, with the right locomotive, they all work fine.