I did the same months ago but with my return and not my Outbound, but the changes were done within a few hrs of my Outbound that simply messed Everything up. In the end LH got it sorted out at JFK .

But I learnt the hard way do NOT redo any of your flights after T-24 to your 1st segment! Once you have flown your Outbound its OK to change your Inbound as it wont mess up the Outbound any longer, any change and CO will reissue the tkt which causes the problems as the Outbound Carrier will see a CX on your res and not the new res as there will be a new tkt # that they would not receive in time

Retkting isnt done in a sec with Shares, so yes they can get the space but till they hear back if it tkted or not LookOut! and most likley what they CXed will be long gone and they wont be able to get it back again

I know it shouldnt be like this but it is like this! Thats why I wont change anything once Im < T-24 to my 1st segment any more