Given the OP stated 'Marylebone to Aylesbury' and 'Aylesbury to Princes Risboro', that was the reason I said good luck with Marylebone to Aylesbury.
The choice there eventually will be one of the following:
- Leave as is with DMUs Amersham - Marylebone.
- Dual electrification to Amersham - Harrow - though I don't believe LUL will be keen on that.
- LUL abandon Amersham - Harrow (highly unlikely)
- Dual voltage units which could work on LUL infrastructure with changeover at Amersham - more likely.