Here's a hint about a good way to convert from Fluorescent tubes to LED's. For now, I'll stick to 4-foot tube fixtures (single or double tube, does not matter) but same goes for other sizes. For background, note that the fixtures and parts for the older T12 tubes and the newer T8's have the SAME tube lengths and tube end pin spacing, so one tube can fit into either old fixture. HOWEVER, the Ballast units are different and so are the wiring details, so a simple upgrade from T12 to T8 tubes CAN be done if you change the ballast unit and re-wire correctly.
To get away from fluorescent tubes entirely but re-use the old fixture takes a different strategy. You can buy simple "drop-in" replacement LED tubes of the SAME length and pin spacing so you just install them into the existing fixture with no changes. The problem I see there is that, since these LED tubes are designed to work using the existing power supply including the ballast units, they are made to work with very high voltage supplied to the tube, and those power systems fail more quickly. The lifetime of such drop-in LED tubes is not great, unlike most LED lamps.
A better path is to use "Bypass Conversion" LED tubes. These tubes again have the same tube length and pin spacing to fit either T12 or T8 old fixtures, BUT they are designed to use different power supply connections. They do NOT make any use of the old ballast units. The 120 VAC power supply wires into the fixture are to be connected directly to the lamp pin sockets so the lamps get that voltage, the SAME as any other LED screw-in lamp, and their lifetime IS longer like other LED's. Obviously this means that you have to change the wiring details inside the fixture so the ballast is NOT part of the circuit, and the line supply wires go directly to the lamp sockets. Since the ballast is not in use, it is probably better to remove it completely and discard according to local rules for electrical waste.
NOTE that there are two small variants of this type of tube. I got ones that use connections at TWO ends of the new LED tube: Hot wire goes to one end, and Neutral to the other end. (Does not matter which is which.) On the tubes there are TWO pins at each end (to lock into the socket and support the tube) and I just assumed that BOTH pins on one end are the same and connected the two wires from the socket at that end together to one power supply wire.
The other variant has the two power supply lines connect to DIFFERENT pins on ONE end, and the pins at the other end are dummies solely there to lock into the socket to hold the tube end.
Personally, I like the colour of lamps at 4000K so that's what I bought. I was fitting out a new garage and wanted general lighting like I get from 4-foot 2-tube fluorescent fixtures. I found four such old T12 fixtures cheap at a recycling centre, modified them to remove the ballast coils and add a white wire from one end to the other for the Neutral connection to the far-end sockets, and connected all up. They look great and provide lots of light, with expectations of long life. And being LED's they DO work properly even at cold temperatures of -35 C (30 below F) unlike fluorescent tube or CFL lamps.