Can i spray over Polyurethane?

Hi guys
Just curious... so I've accidently used an oil based polyurethane spray to finish my guitar, prior to doing so it looked great. Now I've realised as the paint is made up predominantly of a white pigment (its light blue)... that its yellowing...and looking like it's spent a significant amount of time in a heavy smokers home.

Question: Can I sand the poly a bit and re spray lacquer over the top then re finish with something other than poly? or will it need to be completely stripped back? or do I re spray and find a water based poly or something? No idea where to go from here.

The usual rule of thumb is "oil and water don't mix". You could try an oil based lacquer, but its a good idea to try the two finished together on some scrap wood first to see if there is any issues.

In short - not really.
By lacquer do you mean nitro cellulose?
If so, nitro plays well with most finishes, as does shellac.
I think you'll have compatibility issues with water based and oil based polys as mentioned.
But it sounds like what you are effectively after is a colour change.
If you want to spray over the poly then you really should use an effective barrier between it and the new finish - an undercoat that plays well with both. And use good quality lie PPG materials or something like it. And yes, there will be plenty of rubbing back.
Alternatively, and always works well, is rid the axe of the poly and start again.

So in summary - my opinion only based on a few failures,
Rub back poly for new poly
No water and oil based mixes - or use a very good barrier between them (oil based can often go over water based ok, but not the other way around...)
Personally - strip it back and get the finish you want - we've all been there mate...

okay, great thanks for the advice boys. As always, was looking for an easy way out but it appears I'm in for a bit of elbow grease coupled with purchasing new paints. Cheers!

There may be other ways , but this way you'll have no issues - so it may even save some time