There will be no problems installing the
patch and continuing with the existing game.
The first German patch made changes that required the game to be started over, but those updates were incorporated into the English version before it was released, so none of the English patches are incompatible with earlier saved games. Some of the story fixes and changes are not retroactive, so will not be applied to your current game, but the technical fixes, additional quicksave, etc will work with current saves. As an example, if you start a new game after installing the patch, a particular NPC in Verdistis with a quest for you to steal a very handy item will take back the item to complete the quest and then sell it to you; in a game started before installing the patch, you need to just keep the item, since he will not sell it back to you if you complete the quest.
There was a potential problem in the 1.33 patch that could reset the world / fog-of-war in games saved in an earlier version. If someone ran into this bug and saved the game after that, the 1.34 patch would not fix it, thus the recommendation not to use earlier saves that were re-saved under 1.33.
I think the only serious problem without a fix or workaround is caused by tossing a teleporter stone into the castle without getting an invitation, talking to Janus and accepting a quest.
The
Marcus-Tingalf-Trevor-Quest is a little elaborate, and can be partially broken by doing things in too unexpected an order. Checking the quest log for additional information as you progress should help avoid that.
There is a more detailed list below, but if you want to avoid spoilers there is nothing you really need to avoid doing (besides entering the castle early) that doesn't have a fix or workaround if you happen to run into a problem.
QUESTBREAKERS AND MAKING DIV LIFE DIFFICULT (contains Spoilers)