If you can bias each tube separately, you should be able to replace just the single tube without causing any harm, and the sound quality should not be materially impacted (depending on how critically you are evaluating the sound). One big advantage to having matched tubes is that they should age similarly over time. That being said, whether you should use matched tubes is (1) circuit dependent, so follow the manufacturer's recommendation on this issue, and (2) dependent on how resolving your system is and whether you can hear the difference using matched tubes may offer, again a circuit dependent issue.
Anytime you put in a new output tube, turn the bias all the way down before you turn on the amp, then gradually raise it as you check against the applicable meter reading. Because pairs of output tubes often interact with each other (thus the reason for using matched pairs or quads), turn down the bias on all the tubes and gradually bring the bias up incrementally in a "round-robin" fashion (i.e, a little for tube 1, then a little for tube 2, then a little for tube 3, then a little for tube 4, then start over again with tube 1).
So, no definite "should" on this issue (its amplifier dependent) and a qualified "alright".
.
Anytime you put in a new output tube, turn the bias all the way down before you turn on the amp, then gradually raise it as you check against the applicable meter reading. Because pairs of output tubes often interact with each other (thus the reason for using matched pairs or quads), turn down the bias on all the tubes and gradually bring the bias up incrementally in a "round-robin" fashion (i.e, a little for tube 1, then a little for tube 2, then a little for tube 3, then a little for tube 4, then start over again with tube 1).
So, no definite "should" on this issue (its amplifier dependent) and a qualified "alright".
.