A base change will be required if you bid for that base. Some people are happy to be FO's in a particular base whilst waiting for a command slot to appear, others bid widely for a command (by widely, I mean at all bases) and get the upgrade much quicker. It all comes down to personal choice.
If you fail an upgrade you will not get sacked. You go back to your old role with a cool off period before you can try again.
If you are thinking of joining to get a quick command, be warned that there is a very large number of FO's already passed the PCLA and awaiting slots. It could be a very long wait. Unless there is an expansion looming.....