I have the exact same job coming up this weekend. Am an amateur with tubes, so follow guidance at your own risk.
On my tubes there was a section glued directly adjacent to a baffle. I removed the section aft of the baffle.
That part is shortened to the desired length. After preparing the bonding areas I want to inflate the tube, to get proper shape. . The "new" section can't be inflated so I want to fold back the glue area and support it with a ring of plastic underneath to get a somewhat manageable round shape.
Align the sections, apply fresh glue, and once ready fold the new section back to normal. More like a rolling motion.
Deflate and suck all air out hoping that the baffle folds inwards, away from the bonding area, apply pressure for bonding, and partially reinflate to prevent unwanted bonds inside.
Say a prayer, wait three days, slowly inflate.
Another option is to do this on a table, with deflated tubes. I'm just not sure if it is possible to align the tubes perfectly twhen doing this o prevent wrinckles. My tubes were build military spec, with an extra inlay on the inside so a bit of leeway there.
If you have a better idea I'd love to hear it!