Is it okay to replace a larger flexible A/C duct with one of a smaller
diameter (but same length)? I need to replace one (approx. 54"
circumference, 14" diameter) and I'd actually like to reroute it
around some attic beams, but it's too wide to fit between the beams.
Will replacing it with a narrower flex duct increase the pressure and
the efficiency? Is this to tricky for a DIY project? What kind of
reducing connector would I need? This appears to be the part of the
duct that sucks the return air (because I can see the tube shrink a
bit when the A/C kicks on.)

The print on the existing duct is hard to read, but it LOOKS like it
says:

14
max positive pressure=6" water
max negative pressure=1" water
max flame spread 25
max smoke developed 60
max velocity= 4000 fpm
R 5.6
Perm 0.1
Permaflex M-KE