How to massconvert filenames from PREFIX_00000000.png to PREFIX_HH:mm:ss.ffff.png

I've a huge number of png files, which are named the following way: PREFIX_00000000.png.

These files come from extracting frames from video files using ffmpeg -skip_frame nokey -i ORIGINALVIDEO.MP4 -vsync vfr -frame_pts true -r 1000 FRAMES_%d.png which outputs files like:

FRAMES_15416721.png
FRAMES_26708343.png
FRAMES_38000000.png  
FRAMES_49291677.png
FRAMES_60583335.png
FRAMES_71875085.png

(I came up with this by this answer. )

PREFIX_ is a fixed string and is followed by eight digits. Till now the video files are 24h max – which brings me to, 86400000ms (8 digits). Maybe the video files will longer – thus I'm looking for a solution which could be changed to any number of digits. These numbers represent milliseconds (Since the beginning of the video file / representing the time into the video).

Now i'm searching for a way to convert the filename from the given format to the format PREFIX_HH:mm:ss.fff.png. (String, followed by Hour:Minute:Second.Millisecond). So a file named FRAMES_49291677.png would become FRAMES_13:41:31.677.png.

Sample values would be the following. (I added examples for >24h as well.)
Input:

PREFIX_1.png
PREFIX_000000001.png
PREFIX_1012.png
PREFIX_00001012.png
PREFIX_12106.png
PREFIX_000012106.png
PREFIX_725463.png
PREFIX_00725463.png
PREFIX_1204242.png
PREFIX_001204242.png
PREFIX_9864336.png
PREFIX_09864336.png
PREFIX_36012486.png
PREFIX_036012486.png
PREFIX_49291677.png
PREFIX_049291677.png
PREFIX_86400000.png
PREFIX_086400000.png
PREFIX_113744568.png
PREFIX_261736874.png

Output:

PREFIX_00:00:00.001.png
PREFIX_00:00:00.001.png
PREFIX_00:00:01.012.png
PREFIX_00:00:01.012.png
PREFIX_00:00:12.106.png
PREFIX_00:00:12.106.png
PREFIX_00:02:05.463.png
PREFIX_00:02:05.463.png
PREFIX_00:20:04.242.png
PREFIX_00:20:04.242.png
PREFIX_02:44:30.336.png
PREFIX_02:44:30.336.png
PREFIX_10:00:12.486.png
PREFIX_10:00:12.486.png
PREFIX_13:41:31.677.png
PREFIX_13:41:31.677.png
PREFIX_24:00:00.000.png
PREFIX_24:00:00.000.png
PREFIX_31:25:44.568.png
PREFIX_72:22:16.874.png

The turnside? This should be done via bash (Ubuntu).

Did anyone of you do sth like this? Or does have a solution?