To change just the SIM number of a SIM, first change the Cell Phone Number to a unique number (by adding an 'a' behind the number)
then click Submit
Then check replace and change the SIM number and Cell Phone Number to what it should be

When replace is checked it will always create a new SIM card. After submit, the old SIM card will be the selected one.

To swop 2 SIM cards
1) free up both SIM cards by renaming the SIM and Phone on both SIM cards (by adding an 'a' to the end of both SIM and Phone)
2) then delete them (not the ones with the 'a')
3) now replace the ones with the 'a' with the correct SIM and phone

Video about:
-SIM Swop
-Change Phone Number only
-Change SIM number only
-Use SIM in another modem

Thanks!