You can buy LED replacement tubes the plug in to the socket and do not need a ballast change. You can install a LED compatible ballast and buy a cheaper bulb or buy more expensive bulbs the do not require a ballast change.

Years ago I bought 8 fixtures for my shop but since they use the cheapest, piece of junk ballast, I had to replace ALL of them within a year with commercial ballast with no issues for the last 8 years. I have been looking at replacing these lights now with LED's and would opt to go with the cheaper bulb and ballast replacement.

Check the bulb packages to see if they are plug-and-play (used with existing ballast) or ones that need LED compatible ballast. And yes - I would change them two at a time.