Got the answer from another forum, was trickier than expected but works a treat;
Code:
=IF(ISNUMBER(FIND("-",A1)),LEFT(A1,FIND("-",A1)-1)+A2&"-"&RIGHT(A1,LEN(A1)-FIND("-",A1))+A2,A1+A2)
where A1 is the 'xxx' or 'xxx-yyy' cell and A2 is the change cell