sed -r ':a;N;$!ba;s/

([^A-Z])/ 1/g'

:a create a label b

N Search all the lines mashed together instead of searching line by line

$! if not the last line, b branch (go to) label a (skip the last line because the last line has a final newline)

s substitution


([^A-Z]) match a newline followed by anything not a capitalized letter. The ( and ) group together the anything not a capitalized letter.

/ 1/' replace our match with a space followed by group 1

g copy the hold buffer contents to the pattern space

Credit belongs here and here and here.