Hello Anna Fitzgerald ,
Thank you for your interest in upgrading.
Do I download new software or is the change integrated in my 2015 program?
When you decide to upgrade your software, you are actually purchasing a brand-new full version. Your current version is in itself NOT upgraded and you will need to download and install the new version. Just to be clear- the difference between an 'upgraded license' and a 'new license' is purely price (upgrades being far cheaper than purchasing new).
If the change is not integrated into 2015, can I still keep using 2015?
To make the transition from one version to the next as smooth as possible, we allow you (freelancers only) to keep your old license for a limited time. This article explains the process is more detail-
Are 2015 memories usable in 2019?
Yes they are- all Studio memories are in SDLTM format- You may also want to benefit from the new features (available since the 2017 release) such as upLIFT FuzzyMatch repair - for more information- https://www.sdltrados.com/products/trados-studio/freelance/whats-new.html
Can projects created in 2015 be opened and used in 2019?
Yes you can. However, if you are still working on a project in Studio 2015- I would always suggest finishing the current project in 2015 and then slowly migrating to Studio 2019 for all new projects that you receive. I would not advocate switching versions mid-project.