I have a CDplayer as well as the cassette player in my car, and I was wondering if its possible to remove and change the CD player only and still be able to keep the cassette etc?

If its stock Toyota CD/Tape player and you want to put aftermarket CD then no, it's not going to work.
It looks like you are really into Tape players, so you could just buy a double-din headunit that plays CDMP3/Tape. I think Kenwood makes what you are looking for.

if you have the tape player w/ cd add-on type that has a seperate amplifier, it's possible . One way to make it work is to buy a aftermarket unit w/ a auxilary input or cd changer input (that you can find the pinouts to make a input plug for it, ie: pioneer)). You'd have the aftermarket unit working as the main deck and the tape deck working as the auxilary. Wiring would be a little tricky. You'd have to connect the tape deck's amp signal to the aux input of the aftermarket deck and use the same acc/b+/ill/etc... wires for both decks and only have the speaker wires connected to the aftermarket deck.

Thanks for the advice, it's just I saw someone I forgot his name in the Selling/Buying area with the stock cassette and an aftermarket CD player installed in his deck, so I started to think it that set up is possible.

it's even easier to do if you have a aftermarket amp powering your speakers
just connect the speaker out of the deck to the high input of the amp and the aftermarket to the low input (rca plugs).

Hi, had the same problem when i wanted a new cd player, change the whole
unit or try to re-place the cd alone, ended up with a very new used-whole unit out of
a import celica, even better spec with mini-disc, no hasstle with installation, just
straight-in, so if like me, you want to keep the car original, just have a look around,
as most like to replace theyer standard audio with after market,