Well I don't know what THEIR answer is, but most issues I've seen with LED replacements is that they are so efficient that they do not provide enough load for the driver circuit to operate properly. You see this a lot with dimmers.
It's better to just get rid of the ballast. If you really prefer not to do that, then you'll need to just try it out. Make sure what you buy is UL certified - it should at least be safe in that case.