Help with converting page size in Framemaker 7.2 Files?

Create an appropriate template for the Letter-sized pages and then import only the Page Layout via File > Import > Formats. You can update entire books at one time this way.

How many files? (if "massive numbers", aftermarket tools would be worth a look)

Is the majority of the content in each file in and/or anchored to a single continuous Flow A?

Is the content sensitive to column width? (which could be increased going to USL, but doesn't have to)

How many Master Pages per file, typically?

How many MPs shared between files? (might allow fix once, Import Formats everywhere else)

How much is on the MPs? (footers, header, marginousities)

How much is on Reference Pages? (if "lots", this argues for resize in place, rather than copy Flow A into new templates)

You can simply redefine the page size in each file, after moving MP content out of danger. But if you don't have a clean FLow A, it could be a disaster.