It's a pity that you don't mention which OS you run on your
Terminal Server, since the answer to your questions depends partly
on the OS used.
To answer your second question first: the only reason (apart from
rather rare exceptions) why a W2K Pro or XP Pro client would be
issued a purchased TS CAL is when the TS runs Windows 2003.
*No* client OS comes with a build-in license. When you connect to
a W2K TS from a W2K Pro or XP Pro client, these clients receive a
free TS CAL from the TS Licensing Server. When you connect to a
2003 TS, all clients need a purchased TS CAL.
In general, you cannot reallocate licenses, and it should not be
necessary either.
If you run W2K on your TS and your TS CALs were installed *after*
SP3 (which included a Licensing Enhancement) was applied to the
Terminal Server and the License Server, then they have an
expiration date of 52-89 days. Licenses that are not used (because
the client does not connect to the TS anymore) will automatically
be returned to the LS after the expiration date. In the meantime,
your new clients will get a temporary TS CAL, which is valid for
90 days. The above also applies to Windows 2003 with "Per Device"
TS licenses.
Note that these time periods are choosen to make sure that your
temporary license lasts long enough for the old license to become
available again. So if your TS CALs have an expiration date, and
you are reaching the end of the 90 day temporary license period
without having TS CALs available, it means that you have too few
licenses and you simply have to buy extra TS CALs.
If you installed your TS CALs on a W2K TS Licensing Server
*before* SP3, then they will not have an expiration date and be
permanently issued to the clients.
These licenses will be lost by a fresh re-installation of the
client OS, or if the client is discarded altogether. You will have
to phone the Clearinghouse to get these TS CALs re-issued.
291795 - HOW TO: Locate a Phone Number for the Microsoft
Clearinghouse
http://support.microsoft.com/?kbid=291795