API has a number of Recommended Practices, which include things like hot work and working under floating roofs, and those should be reviewed as needed. Generally, it wouldn't be any harder to remove a floating roof than to do other repairs in the tank.
The floating roof is not normally considered to add any stability to the tank itself, so there would not be an issue there. One thing to check is that without the floating roof limiting the fill depth, it may be possible to fill the tank to depths for which it was not designed.
Presumably, you have looked at operational and emissions aspects already.