I work at Lowe's in the electrical department and get this question often so did some research. It seems to be a gray area. Older fixtures may work with the bypass while newer ones with electronic ballast may work as is. But there is also the possibility they wont. Seems there is a cut off period with the electronic ballasts that 100% work, but there was no date I could find. If you only are replacing a couple I usually tell people they may want to consider new fixtures. Jim
There are three different classes of LED replacement tubes.
Type A are a direct swap "plug-n-play" replacement but all the ones I've ever seen will only work to replace T8 bulbs in a fixture with an electronic ballast, and the ballast must be left in the fixture. Easy replacement, but more points of failure. All of the GE brand LED tubes that are carried in all the Lowes around here now are this type. Not sure about other areas.
Type B tubes are ballast-bypass tubes that are direct wired to the sockets/tombstones, and are available in single-end or double-end power. These are usually a better replacement option for T12 fixtures since there are few, if any, magnetic ballasts from old T12 fixtures that will work with LED tubes. The single-end power tubes use non-shunted sockets (which T12s use) and supply line and neutral to one socket, while the other end is a "dummy socket" that exists just to hold up the bulb. The double-ended power LED tubes required shunted ends with line going to one socket and neutral going to the socket at the other end. If switching from T12 bulbs with non-shunted sockets, they would have to be shunted with a jumper wire. If switching from T8s, they likely already have shunted sockets. None of the Lowes around here carry either type of Type B tubes in stock, but the double-end power tubes can be special ordered (although only in multiples of 20). Single-end power tubes can not be ordered. Utilitech used to make one Type B tube that was carried in the stores but I doubt they're long gone at this point.
Type C LED tubes run on low-voltage fed from an external driver. I don't have much experience with these, nor have I seen them in local stores.
If you want a list of model and item numbers for the ones that can be ordered, let me know. I rarely get customers ordering them to replace T12's because I can't order less than 20 and they only need a few, so I just recommend a new fixture.