exchanging the layers source footage with Alt + Drag new footage on top of Layer works fine ... but when the layer had a mask, the mask shape changes ... probably because the layer-updating comp has another scaling, though it didn't transfer scaling A to scaling B ... scaling B was just much higher. Locking the Mask unfortunately didn't help.
Any hint around how to keep the original Mask shape on the Layer ?
You're replacing a layer with established horizontal & vertical dimensions and pixel aspect ratio with one that has different properites. You can make a comp with the same settings as the original layer, put the new layer inside it and THEN use it when you replace footage. Your masks will be just right.
k thx for the solution Dave. Was already asking myself if there is something like a freeze button to bring all values to neutral, while keeping the properties as is. Pre-composing in the same size basically does exactly that, just manually.