Function to populate a field in a report with corresponding data from another report

I have a form and report (A) in which I forgot to add a field called 'User1'.

I have another form/report (B) that has the the field 'User' in it.

They both have a field called 'Patient ID' that is common between them.

I would like a function script that I can execute as a one off that will do something like this:

for each record in FormA where 'FormA Patient ID' = 'Form B Patient ID'

update 'FormA User1' with 'FormB User'

There are 15000 records to update.

Would this be possible?

Thanks in advance