The sound of your guitaramp comes from various aspects: The design of the amp, the type of valves being used and the speaker. Changing the speaker of the amp can have some interesting results: A stiff amp may now sound more responsive, the amp may break up in a different way, the brightness of the amp may have been tamed. These are just a few aspects I mention, difficult to predict how your amp will sound, just giving you a general overview of how the sound of your amp may have changed.
In case you use amphead and separate speakers, changing the kind of speaker you use will be very easy: Just plug amphead into different speakcabinet and listen to the results.
Using a combo? Some combo amps have a jackplug on the lead of the speaker, this jack plug is then connected to the speaker socket of the amp which lets you connect speaker to the amp. If your combo does have a jack plug connected to the speakerlead connect this jack to a different amp: You are now using the speaker of your combo to connect to a different amp. Often the speaker lead found on the exhisting speaker of your comboamp will be short. You may be able to plug your speaker into a connnectorbox which will let you plug speaker into box and then let you connect this box to a larger speakerlead which may be able to reach your amp. In case you are not really sure how to get this connection done, speak to an engineer or a friend who is well-versed in technical details as such.
Using Speakers You Know:
Trying different speaker onto various amps may work best if you use speakers you know, since you know their character and will be able to notice the differences in sound straight away. Working in this manner also avoids you having to buy new speakers, speakers you are not sure what they sound like.
Overal, this experiment will open up your ears to particular characteristics of your amps, observations which will make you understand your amps better.
Have fun and hope to catch you soon again,