My issue is different. I am OK with syntax, IDs etc. My issue is; is there a way to batch convert a whole load of INSERTS INTO/SELECT parts of SQL statements into UPDATE/SET parts of SQL statements?
Yes, that should be doable, but don't expect anyone to serve you a solution.
First of all, you need to choose the language for the task. C#, Python, Perl and Powershell are all good candidates. T-SQL is most emphatically not. You will need to do a load of regular-expression matching to do this.
There is also a semantical challenge: how to know what are the key columns of the tables, and thus go the JOIN clause rather than the SET clause. This can be deduced by reading metadata from the system tables. It is not trivial to do from the source code alone, unless you have a really simple rule as "the first column is always the key column, and there are no composite keys".