I would imagine that replacement would be pretty simple since LED auto
lamps are a commodity item (as opposed to the specialized modules used
in newer high-price vehicles). My only concern would be ensuring that
the light output and light spread from the replacement would be
compatible with the application -- turn signals which can only be seen
from +/- 1 degree off-center or only in the dark of night would be of
little use to me.

One established source of LED automotive bulb replacements is
http://www.superbrightleds.com/carbulb-notes.htm

Turn signals may flash twice as fast as normal because of the lower
current (the car thinks the bulb has failed).  Most aftermarket
replacements for sidelights are pretty poor off-axis and may not meet
vehicle lighting regulations.  The best ones for stop/tail lights are
OK, because powerful red LEDs are well developed.  Get some with LEDs
which face the reflector as well as the lens.