1. you must know what safe mode does...
actually some providers use safemode for security issues.. if on your server php is in safemode then: php cannot create directories or execute shell commands.. it is a php.ini setting.
if you are not sure how secure your php script is.. then safemode is better because there are less chances to get hacked...
ATTENTION: safe mode does NOT exist anymore as of php > 6.0
to check if your server is in safemode.. create a php file and call it safe.php
the content of the file should be:
Content visible to verified customers only.
upload to your server.. and run safe.php
it should print out the result..
2. in gallery the safemode bypass switch actually does only 1 thing..
it stores ALL your image files in 1 directory only that has been created by you.. so php does not need to create directories by itself..
so IF your server is NOT running in safemode.. you can use both gallery settings.. though it is better not to use the safemode setting.. but both settings should work.. (safemode setting limits the amount of images to ~ 7-8000 images and server gets slower the more images you have)
IF your server is running in safemode, then ONLY the safemode bypass setting will work.. (Otherwise you will get errors when trying to upload images)
I tested changing settings with about 400 pics.. it worked.. i did not loose any picture.. (I changed in both ways) But attention, like everytghing there can always be an error somewhere.. (especially if you have replaced images.. there was a bug with replace images in safemode.. so there allways can be a problem)
What I suggest proceed the following way:
1. move to the new server.. once you are there make sure everything is working.. ALL images should be exactly like on old server.. in the file directory.
2. backup the file directory with ftp to your harddisk
3. backup the images table with phpmyadmin (table ppgal_images)
4. go to ACP and switch the safemode bypass in gallery section.. it should start working and move images into subdirectories..
this can take a while.. (400 images ~ 5 to 6 minutes)
Once its finished.. all images should be in subdirectories named by the userid of the owner of the image..
if there was a problem.. you can allways restore the ppgal images table and copy the images back to the directory.. so no harm is done..
hope that helps
PS: if YOU are the ONLY ONE uploading to the gallery.. then there is no difference between both settings.. because in safemode mode.. all images are in 1 directory (files/*.*) and in not safemode all images will be in your directory.. (files/1/*.*)