I don't know how you could book it at 55 minutes if it weren't legal. Since you're coming in on LH shorthaul, you're likely to get a gate, rather than a stand but it'll be one of the Schengen gates. You'll have to go through passport control and at least two security checks before boarding, so it'll be close.

You can only make the change if you have a changeable ticket, otherwise a charge will apply. Note that you cannot generally go standby on non-changeable tickets in Europe like you can in the US for free--you'll be charged the fare difference if you want a different flight.