Converting Periodic AST to Periodic Standard Occupation Contract

Tenancy Types and Management

I am going through the process of creating a Periodic Standard Occupation Contract, and find it quite confusing.
I am starting with the Model Periodic Standard Contract supplied by the Welsh Government. I am then striking through any non-applicable clauses and modifying those where applicable with the changes in capitals. I am then adding Additional terms that were reflected in my original AST. Is this the correct approach? There seems to be a lot of terms in the model contract that does not apply to an existing tenancy, but more applicable for ones which are about to start. I have also looked at the Standard Occupation Contract created by the NRLA and this does not follow the structure of the model contract.
Also not sure about how to date this. There is an occupation date - should this be the original date the Tenant moved in, or the date of the latest AST? Also the date when rent starts, should this be the next payment date, or another date bearing in mind that the rent amount will have changed between dates?
Thanks.