Can I replace the old disc HD in my MacBook with an SSD? That is, are they interchangeable, now?
Yes, you can replace your HD with an SSD. They are mainly packaged in the same form factor to fit as replacement. I would look on Pricewatch.com to find the best price for a size you need at a reasonable price.
Yes. Depends on how much storage space you require. SSD's are expensive. I bought a Seagate Momentus 750GB Hybrid for my MacBook. It combines SSD tech with old platter tech, so you get fast speeds and large storage for under $100. I prefer keeping my DVD drive; otherwise you could convert the DVD slot to a second HD slot for a large mechanical hard drive and use the primary HD slot for a true SSD to keep costs down but get the highest speeds possible.
For my desktop machine, I swapped in a true 240GB SSD, and plugged in a large external mechanical 3TB HD for lots of space.
Extremely easy to swap in SSD storage, same form factor. Cloning software is readily available to clone your old HD to a new SSD; takes a minute to physically swap in the new drive.
I was actually thinking more in terms of shopping for a new machine.
However, SSDs are designed to be dropped in as replacements for existing rotating drives. See this link, for example, on upgrading the MBP.