According to the chart you should be able to but YOU SHOULDN'T.  If you want gunners, you should level dedicated gunners not flip flop between the 2 classes as you lose accuracy and reload during the torpedo stages.

I understand that, but its really just the time expense of leveling up two new gunners to be able to have a functional BB.  I didnt have much foresight when first making my fleet/navy.  

yes he's right, you could sell those torpers or trade it for exact level gunners...

classing it back to gunners will not be good since it lose 5 base stat on acc and rel when you class it as torpedo.

And it's not just a matter of losing for example +3 for a few levels because of missing a classing.  When you turn them into torpers, as noted it's a minus 5 acc/rld which is a permanent loss.  That would mean that even if you had elite gunners, you've turned them into 8/6 gunners.  Since they are torpers, I assume it was a torpedo high stat... meaning those gunners would be the lowest of lows in terms of ability.

Just sell or trade the torpers for something fair(likely a little lower level) and work from there.