If the tenant has offeed to pay 6 months advance rent that is not going to effect if the tenancy is an AST. However, you should ensure the rental frequency is still the rent to be paid PCM. So you accept the 6 months rent, when the 6 months are up, the rent frweqency contractually is still PCM.
Should you need to serve a section 8 notice in the future the frequency of payable rent can have a detrimental effect on the notice period.
So ensure the frequency on the tenancy for the payable rent is PCM or weekly.