UK Landlord Forum / Life

Advice please. I have recently purchased a business and I believe I am obligated to issue new contracts to all service users. However I wish to increase the rent for 1 service user. Is the old contract (signed with previous owner) void, and if so, am I within my rights when I'm issuing the new contract to stipulate a reasonable increase. 
I'm assuming this is not like TUPE laws in that old contracts transfer across irrespective of owners.

Any advice will be greatly appreciated.