You may be interested in Number::Bytes::Human or Number::Format from CPAN. Never used the latter personally but it looks like it has some support for "unformatting" numbers which you could use to parse your input and then reformat things back out with whatever different suffix and/or commaification. I have a shell function using the former that I keep on hand for hyoomaning up arbitrary numbers in the shell.

format_bytes () {
  perl -MNumber::Bytes::Human=format_bytes -lE 'for( @ARGV ) { say for
+mat_bytes( $_ ); }' "$@"
}
[download]

sub rc {
    my $n = int shift()/1024;
    $n =~ s/(\d+?)(\d{3})$/$1,$2/;
    return $n;
}

s/([^:]+:)\s+(\d+)\s+KB/
    sprintf "%-14s%10s MB", $1, rc($2)/e;

s/^\=+$/"=" x 68/e;

my $IMCfmt = "%-17s%17s%17s%17s";

s/^(Name)\s+(Mem Used)\s+\(KB\)\s+(HWM Used)\s+\(KB\)\s+(Cached)\s+\(K
+B\)/
    sprintf $IMCfmt, $1, $2." (MB)", $3." (MB)", $4." (MB)"/e;

s/^([APPL-|DBMS-|FMP_|PRIVATE|DB-]\S*)\s+(\d+)\s+(\d+)\s+(\d+)/
    sprintf $IMCfmt, $1, rc($2), rc($3), rc($4)/e;
[download]

Here is a "hack it with regex" approach:
This doesn't exactly replicate your formatting. But to my eye, it looks "close enough".
I just started adding one regex at a time until I got something "close to desired".
Mileage varies. I used "just a complicated enough" regex for your example. I didn't consider other examples.
Again, mileage varies. But maybe you will see something useful in this hack.
Also, I considered running 5 regexes per line to be of no performance consequence at all.

ADDED: One complication that I did see was, what if Name like "DBMS-db2pb13345" appears? A more complex regex would be needed to skip that. Also from your problem statement, it sounds like sometimes this might need to go KB->GB instead of KB->MB? I'm not sure that that would mean. "Commifiying" the numbers adds a lot of readability. Your sub for that is fine and right out of Perl Cookbook!

use strict;
use warnings;
use POSIX 'ceil';

#-- Format memory in MB from db2pd -dbptnmem
#-- Run as db2pd -dbptnmem | perl -nl db2mem.pl
#-- or  perl -nl db2mem.pl db2mem.PB1

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}

while (<DATA>){

   print, next if ($.==1); #don't process first line
   
   s/KB/MB/g;                              # all KB->MB
   s|(\d{2,})|commify(ceil($1/1000));|ge;  # move decimal point
   s| (\d+,)|$1|g;                         # allow for one comma in al
+ignment
   s|   (Mem Used)|$1|;                    # adjust columns to left 3 
+spaces
   s|===||;                                # shorten separator 3 colum
+ns
   print;
}   

=OUTPUT
Database Member 0 -- Active -- Up 26 days 13:26:26 -- Date 2020-07-10-
+10.11.32.332869

Database Member Memory Controller Statistics

Controller Automatic: N
Controller License Limit: N
Controller Limit Enforced: Y

Memory Limit:        26,598 MB
Current usage:       25,769 MB
HWM usage:           26,004 MB
Cached memory:       3,353 MB

Individual Memory Consumers:

Name          Mem Used (MB) HWM Used (MB) Cached (MB)
=====================================================
APPL-PB1                197        602       39
DBMS-db2pb1             211        211        5
FMP_RESOURCES            23         23       21
PRIVATE               1,378      1,425      844
DB-PB1               23,962     24,334    2,445

=cut

__DATA__
Database Member 0 -- Active -- Up 26 days 13:26:26 -- Date 2020-07-10-
+10.11.32.332869

Database Member Memory Controller Statistics

Controller Automatic: N
Controller License Limit: N
Controller Limit Enforced: Y

Memory Limit:         26597404 KB
Current usage:        25768448 KB
HWM usage:            26003008 KB
Cached memory:        3352576 KB

Individual Memory Consumers:

Name             Mem Used (KB) HWM Used (KB) Cached (KB)
========================================================
APPL-PB1                196352        601792       38272
DBMS-db2pb1             210752        210752        4736
FMP_RESOURCES            22528         22528       20736
PRIVATE                1377280       1424640      843968
DB-PB1                23961536      24333568     2444864
[download]