Nearly all boards have a 'reset CMOS' jumper to wipe the RTC, and still other boards have had the contents lost when the battery goes flat. It doesn't cause any harm, unless you have an old hard drive that needs manual BIOS settings, in which case you only have to put the same settings in or you might have to reformat the drive.

If you're swapping RTCs, it's probably a good idea to either reset the CMOS using the jumper, or at least go into the BIOS setup and load the defaults after putting the new RTC in, just to avoid the potential situation where the old and new BIOSes use the same checksum, and the new BIOS accepts the existing RTC values, which aren't valid for the motherboard.

Unlikely to cause major issues, but if there are things in there like CPU multipliers then you may not be able to boot until they are reset, and the BIOS reset jumper on the motherboard is probably the best way to do that.

Remember that the BIOS reset works by putting a jumper on it, powering up the system, it won't POST, power it down again, then remove the jumper. If you just connect the jumper without powering up the system, it won't erase the RTC.

You will surely want to "clear CMOS Data" upon transplantation of the new RTC. The orginisation of the BIOS on the old MB will likely be different to that of the new MB. Keep in mind that if you are using a Dallas 12887, the versions without the 'A' suffix do not have a jumper option to reset the CMOS.

Cheers guys.

I wasn't sure about the stored settings and the effect it might have being put into another system... but if I can clear the CMOS, swap RTC's to test, clear CMOS, and swap back to initial settings... then that's great.

These Benchmark versions with the 'AMT' suffix are apparently compatible with the Dallas 12887's that have the 'A' suffix... so I should be ok to go ahead and switch them for testing.

I just hope the non-working system will now work... and after switching them back, the working system will continue to work, as if nothing ever happened.

Be sure to note the existing CMOS settings on the working system, that way you can set the values back to where they were as the 'known good settings'.

Yea, CMOS settings will be saved thanks.

I have to replace (coin cell mod) on one of those Dallas RTC's without the 'A' suffix, it's on a Systempro motherboard... the plan is also to overclock the FSB by de-soldering the OSC beside the RTC and replace it with a socket so the crystal can be interchangable.

Just thinking, if it would be more wise to buy a socket and a couple of new batteries?
That way, it can be changed really easy, if it dies.

Ebay should have a lot of RTC batteries. The coincell is cheaper for shure.
Though a new Dallas would look better I guess.