I have a 48 tooth main ring with 165 cranks. FD is Sora double and rear is Tiagra short with Shimano 7 freewheel. If I can put this 34 ring on it gives me sufficient range for the next few months and time to evaluate before any larger purchase of cranks. (likely I would go to 170 0r 172.5) I'm using Diacompe bar end friction shift.
Is it necessary or desirable to replace large ring as well?