Repeating bits from above
OP has a square taper chainset. Very likely 130mm BCD so won't get a 34t inner ring on that.
For a 48t/34t they'll need a new chainset, and that may or may not be OK with the current BB spindle length. I would look to fit an 'MTB' triple (from my 'box' but widely and cheaply available second+ hand) and use only the outer two rings (eg 42/32). If rivetted, check chainstay clearance of small (unused) ring.
With a 48t large ring, assuming the current chain is the correct length (with the 52t), shorten chain by one (1") link, but it will roll on entirely happily at current length.
Dropping from 52t to 48t will mean dropping the FD down maybe 8mm on the seat tube to keep the cage 1mm above the large ring, for best shifting (drop 2mm per tooth).
There is zero reason to fit a new chain if you're changing chainrings.
OP could 'upgrade' to a 6sp freewheel (assume friction shifting if "vintage") but the largest sprocket is normally 28t. Theirs might be 14-25t 5sp block, so that'd get an extra 12% lower gear.