Search and replace using prename or sed - bash

Question

I am trying to perform search and replace in file names as following:
$ tree
.
├── a_a
│   ├── a_b_c
│   ├── a b c.mkv
│   ├── b_b
│   └── b_c_d
├── b_a
│   ├── a_f_r
│   ├── c_d
│   ├── f_r_e
│   └── r r
├── c_r
│   ├── d_f
│   └── r_a_s.mkv
└── d.mkv
I want to replace the underscores in the file and folder names by spaces. And the way I want to do this is by replacing the underscores in the base names of the files and folder present in the inner directories first and then move up so that the path I am recursing still exists in the next iteration, since if I rename the upper layer directories, in the next iteration the path to access its inner directories and files will become invalid.
I know I can recurse over files using the find command. Now I want to use a tool to perform the replace operation starting with the files inside and then moving outwards. I don't have much experience in writing regex but I think we may be able to do this using grouping in regex, but I am not sure so plese help.
Till now I have been able to figure out that we can use regex groups to access some parts of the file name. To be more specific, we can get the base name of the folders and files using following regex:
rename -n 's!([^/]*\Z)!uc($1)!e' ./*
Using above regex in the rename command we can convert the base name group to uppercase and I want to know how can I replace the underscores in that group to spaces.
PS: Also I know some of you might say this is a duplicate question, but please read it again, I have researched a lot before asking the question and could not find this specific question anywhere.

Answer 1

#!/bin/bash
find -depth | while IFS= read -r fn; do
pnew=$(dirname "$fn")
fnew=$(basename "$fn")
if [[ "$fnew" =~ "_" ]]; then
new="$pnew/${fnew//_/ }"
echo "$fn -> $new"
mv "$fn" "$new"
fi
done
Remarks:
The -depth argument lets find traverse the directories depth-first.
The dirname/basename-split prevents directories from getting renamed along with their children. Only the leaf file/directory may be renamed at a time.
Everything is quoted where needed to allow for spaces in filenames (including incoming filenames).

Answer 2

#!/bin/bash
# man find, search for -type
#
# these are other types:
# b - block special, c - character special, d - directory, p - named pipe
# f - regular file, l - symbolic link, s - socket
# Move directories first, then everything else
for TYPE in d f; do
for NAME in $( find . -type $TYPE -print0 ); do
if [[ $NAME =~ [a-z] ]]; then
NEW_NAME=$NAME
NEW_NAME=${NEW_NAME//[\_]/-} # Change '-' to ' ' if you insist on spaces
echo "renaming '$NAME' to '$NEW_NAME'"
mv "$NAME" "$NEW_NAME"
fi
done
done