When my pipe dropped out of the flange and into the holding tank, well almost, but did separate from flange. I found the pipe into tank is held in tank by a rubber grommet seal. If you pull hard enough you can move the pipe up and down but do it slowly.
If your going to move flange I would find a possible off set from pipe to flange so you can use same hole in tank. If you brake the seal to tank be prepared to have some bleach handy because if your dumps miss getting into tank you will need it.
Thats the other half of the story when pipe drops out of flange.