if i activate my "interrail global pass" (flexi pass) by entering the departure date, can i change it later?

Until 23.59 the day before the start date of the validity of the pass you can change the start date. However, the advice from the experienced travellers is to avoid to activate the pass in advance. Do it just before boarding the first train of your travel in case your plans change. This is very important with a flex pass as you can loose travel days.