Can you change UK Self Assessment Tax return after it's been filed?

My husband is self employed and does self assessments on HMRC website.  Well he's deducted too much this year...in that now he falls below the £18,600 needed to sponsor me to move back to the UK.  Despite my excellent salary here in the US and previous work in the UK, they go off his pre tax profits.

He just completed his last financial year return on HMRC website.  If he had not deducted for things like car and phone expenses, he would make the threshold.  We were advised to have an accountant do an official audit and maybe the numbers would work then.

Our fear is that this would raise a red flag with HMRC that could cause us headaches in the future.

Any advise or guidance as to where I could find it this is even allowed and if so if it will cause us trouble in the future?

Thanks

I would check with an accountant.  I've done it in the reverse direction.  I changed my self-assessment after it became clear that I had filed it and left off some big deductions that I could have claimed.  My accountant filed things and they took off the deductions, so I imagine you could maybe do it in reverse--don't think I'd want to do it myself though, call an accountant.

Can you change your assessment? - yes. See http://www.hmrc.gov.uk/sa/correct-repay.htm.

An accountant, perhaps one specialising in tax, would probably be worth his or her fee (GBP 250 is our estimate) and will need to consider the definition of 'pre-tax profits', paying attention for instance to whether capital allowances and pension premiums should be deducted to get to 'pre-tax'. One can defer capital allowances. 

It won't red flag HMRC. They might send you a note saying thank you for paying more tax (Brit humour).

(You may also want to get hold of an 'incoming checklist' on moving US to UK and run through it to reduce tax and make it easier to control and manage your money.)

You would usually want a US/UK dually qualified tax adviser (not necessarily an accountant) - but in this case I would consult an immigration lawyer first as it is a question of immigration law, not tax law.