You can leave your old RAM in place. As others have noted, this may have a marginal impact on speed, but it definitely will not be any slower than it is now provided whatever RAM you slot in there is at least equal to or greater than the speed of what you already have. The entire bank of RAM will run at whatever the speed of the slowest module is. If you’re lucky, your existing RAM modules will have a sticker on them describing the speed.

The speed is the number like DDR4-2400, -2666, -2933, -3000, -3200, etc.

In the bad old days of single data rate RAM (We’re talking like '80’s and '90’s, here) the speed rating was literally the bus clock speed, in megahertz, that the RAM was rated to cycle at. Nowadays with DDR RAM that communicates more than once per system clock cycle, the speeds are in “megatranfers” and are to a certain extent both theoretical and a marketing description of the absolute ideal case scenario.

In any event, higher numbers = faster, although in reality your memory speed is unlikely to make much if any observable difference in the performance of your computer since most ordinary desktop or even gaming tasks are not especially memory speed dependent and even if they are, even slow modern DDR3/4/5/whatever RAM is still, in real objective terms, quite fast to begin with. Also, your processor and board almost certainly also have their own maximum RAM communication speed, which may be less than the fastest RAM modules you can buy. Thus buying RAM any faster than whatever your board/processor’s maximum is will be pointless anyway.

Not having enough RAM to do whatever you’re doing will force the system to use the SSD or hard drive as virtual memory, though, which will instantly make it dogshit slow.