Yes you can sign up for subscription (still $199/month last time I checked). Yes you can cancel the subscription after any number of months.
BUT, just because you have an active paid-for subscription does not mean your car instantly downloads an FSDb software load. Generally the FSDb software loads have base weeks well behind the software loaded on most non FSDb cars. So many customers deliberately avoid installing new non-FSDb software releases and wait for a new FSDb release to show up which has a base week higher than the software installed on their car. THEN subscribe, and hope that pretty quickly you'll actually get a download of the software, install it, and activate it.
Regarding the base week matter:
The most widely installed software as of today's reporting by Teslaupdates.org is 2023.12.5 (i.e. week 12 of year 2023)
But the widely available FSDb software is 2022.45.15 (week 45 of year 2022) and the latest FSDb software reported sent to any end users (and that in small numbers) is 2023.7.5.
As Tesla still does not routinely install "older" software on a vehicle, you can see the problem.
Less clear than the "older" problem is the "not a factory build problem".
Usually Tesla has not distributed FSDb software to cars currently running the build with which they left the factory such as 2023.12.100 (last field ends in 00).
The prudent subscriber with a new car might follow this sequence:
1. Install the first new software you are offered.
2. Decline to install further software while you wait.
3. Wait until there is a broadly released FSDb software version with a higher number than your software.
4. Subscribe.
5. Hope you get your FSDb download soon.
6. Install it. (but continue not to install any non-FSDb version you may get offered)
7. Activate FSDb on ti.