score:3
TL;DR: You can "upgrade" during a cruise.
There's no such thing as upgrading during a cruise with Tallink Silja. But what you can do is to get another cabin and surrender the keys to your old cabin (if you want). The result is the same as for an upgrade.
The prices are fixed and are full-price with no option of getting it cheaper or paying with membership points. For example the suite from Turku to Stockholm (one way) will set you back around 500β¬ while you can get it both ways for just 40β¬ as a member when using points.
If you just want to get rid of your snoring room mate there are free earplugs offered at the information desk.