Increasing rent during fixed term lease

If I agree to install air-con in a rental property in exchange for an increase in the weekly rent from the tenant (with a fixed term agreement already in place and not due to expire until 2021) then is there a way to do this without having to organise signature of a new lease?

e.g. can you just get an agreement in writing (by email) that the tenant agrees to an increase of $xx per week in rent in exchange for air-con

Or is there a more formal way to approach amendments to a lease that do not involve re-drafting the lease from scratch?

This is in QLD.

 

You will probably get away with getting a written agreement, but you really should draft a new lease.

 

An alternative to altering the lease is to ask for a contribution to the cost (That would equal the rent increase over the rest of the current lease) then renew the lease at an increase when the time comes.

In Vic rent increases are not possible right now so you need to think outside the square. Just be careful in how it's worded and drafted that's all (I understand you're in QLD so you may not have rent increase issues currently)