Change 2 values in a sting

I have the following string '2014/15' passed as a parameter to my procedure and I need to increment the "4" and "5" by 1 so I end up with '2015/16'.

Obviously the string is a VARCHAR so I can't use any normal arithmetic functions, nor DATEADD. The parameter must be passed in as is and I must derive a year above and a year below as well as using the parameter.

To simplify, if you feed in '2014/15' then in my SP I will need to use '2013/14', '2014/15' and '2015/16' in various parts or the SP all derived from that 1 string.

Any ideas?

Thanks