IS this safe to install?
I'm assuming that the system already installed. then, yes. it's save to install them. though, you might want to reinstall solydx-info after the update.
Is there (or will there be) an update process from SolydXK Jessie to SolydXK Stretch? I thought since KDE changes from KDE4 to KDE5 Plasma this is not possible, the differences are too huge. It is a completely different system.
yes, there'll be one. if you're on vanilla debian you can jump into stretch now but if you're on solydk you'll need to wait until the team sorted out the update process.
the update process will be magically handled by both debian and solydxk team so you can expect a smooth transition between jessie and stretch.