Note - this article refers only to versions of Kartris before 2.0. Kartris is now free and open source.
Free License
The free, limited licenses issued with the default Kartris zip will typically have a long validity (i.e. 100 years+), but limited upgrade support. However, an up-to-date valid free license is included in the latest Kartris zip. If upgrading a free licensed site, you will generally download the latest Kartris zip; so just ensure you include the license.config file from this, and everything should work correctly.
Unlocked License
If you have purchased a Kartris license, this will typically include a year of free upgrades. This means that the license will work with any core KartrisCore.dll (located in the /Bin folder) that is issued up to a year after your license purchase. Beyond this, you can purchase an upgrade that extends upgrade support for your license for a further 12 months.
If you purchase technical support, this will typically also include a 12 month license upgrade for one site too.
Before upgrading, you should check your license to confirm it is valid with the latest Kartris code you're intending to run. Open up the license.config file and find the following lines:
<startdate>2011/03/24</startdate>
<enddate>2111/03/24</enddate>
<lastupgradedate>2012/03/23</lastupgradedate>
It is the 'lastupgrade' date that you need to focus on. Check this against the issue date of the Kartris zip you're upgrading to, and ensure the 'lastupgradedate' is later than the issue date of the new Kartris. If you try to install a new Kartris with a license that does not support the upgrade, you will get an error.
Note that the upgrade date restriction actually applies to the KartrisCore.dll specifically. So even if your upgrade period has elapsed, you *may* find that other files from the latest zip can be used in places, though there is no guarantee of compatibility with older versions of the core.