Those are two different questions really.
For the first question:
MATLAB Student Version licenses come in two versions, one of which is only valid for a term, and the other of which is indefinite for as long as the owner is enrolled in a qualified post-secondary education institution. The first type will expire, the second one will not. (We can tell that you are not using a Student Version license because those licenses do not have Software Maintenance Service)
The other kinds of licenses, Home, Professional / Commercial, and Academic, are all indefinite and do not expire. However, some sites do negotiate fixed-term licenses with Mathworks. Custom licenses are custom rules.
For the second question:
Toolbox licenses must be purchased against the same version of MATLAB that they will be used with. You cannot, for example, purchase an R2015a toolbox unless you have an R2015a MATLAB.
Mathworks does permit purchasing a toolbox for a non-current release, but only for a small number of years (I do not recall at the moment if it is one or two years.) So if the current release is R2015a and your version is R2014b, then Mathworks would permit you to purchase the R2014b version of the toolbox -- but they would not, for example, permit you to purchase the R2012a version of the toolbox.
If you need to buy a toolbox for a release that is older than Mathworks permits to be purchased, then Mathworks requires that you either purchase MATLAB again or that you back-pay all of the Software Maintenance that you skipped. If I recall correctly even that has a limit of 4 years, after which you would have to re-purchase MATLAB (and all the toolboxes you wanted to use with it.)
