Actually this one is a simple operation. There are just few things to lookout for and a bit of soldering knowledge.
What to look for:
- Speaker impedance - if not written like in my case, measure it with an Ohm-meter
- Speaker size - although may look tempting to put a larger speaker if you change it anyway, usually there is not quite enough space to accommodate a larger speaker, and you have to keep in mind that if it fits in the box then you need also to route the speaker hole (makes no sense to have a larger speaker just to have it covered by wood, right!?!?)
- Mounting holes - if you are not in a mood for a bigger effort to modify the combo (including complete disassembly of the cabinet box) then look for a speaker which has the same mounting pattern as the current (no biggie, they are quite standard so you will find very easy one that fits)
- Power rating - keep in mind the power rating of your combo, do not put a speaker with smaller power rating than the amp, because you can destroy your speaker, and even if survives, will not sound good once it is over the supported power rating. The other way around is ok, 100W speaker in a 50W amp is ok, it may not sound so distorted like a 50W speaker at the maximum output but this is a positive aspect - gives you a cleaner sound.
A few considerations:
Speaker impedance - this one is a very important value. If you use a lower impedance speaker you may burn your amp. Let's say you have a 16 Ohm speaker (so the amp is expecting a 16 Ohm load) but you decide to change it with a 4 Ohm speaker because somewhere on the internet is stated that the high quality studio monitors and audiophile speakers use 4 Ohms. Without going into physics and using the language of my grandma - your amp is expecting a 16 Ohm load, so it outputs accordingly, but there is just a 4 Ohm speaker, so the unused load keeps accumulating in the amp, overheating some components which will be a good start for a failure (with or without flames).
The other way around is ok for the amp, not so ok for the sound - you have a 8 Ohm amp but you put a 16 Ohm speaker inside. While in theory (just electronics) is ok because the 8 Ohms produced by the amp are consumed by the speaker, the remaining difference can be considered like a resistor, which acts like a filter, so you end up with some filtered frequencies and also a lower speaker output.
Get the same impedance! Is ok for the amp, is ok for the sound.
Changing from one speaker to two? Or changing a two speaker cabinet? Then keep in mind the impedance and the connection type. 2 x 8 Ohm in parallel results in 4 Ohm final load, while 2 x 8 Ohm in series results in 16 Ohm final load.
Steps:
1. Open the enclosure
2. Remove the old speaker
3. De-solder the wires
4. Re-solder the new speaker - pay attention to the polarity (+ and -)
5. Screw the old speaker in place
6. Close the enclosure
7. Enjoy!