Stretching a boat seems an expensive and troublesome way of gaining more room.
Selling your boat and buying another would be easier,however,I do understand your hesitating to do this. Buying a boat is fraught with dangers. You can have your prospective boat surveyed inside out,but like taking up with a new woman,you don't know what you have got untill you have lived with her for a year.
I wouldn't have bought my present boat (not at the price I paid at least) because of all the faults that I discovered in the first year of ownership,that were not found on the survey.
If you are well endowed (with money) then a stretch would be ok,if it is done by skilled people and the really labour intensive part, the fit out,is done by equally skilled people.
Good luck whichever route you take.