Does an ECU can switch over to MAFless once the MAF is maxed out. Mine it doesn't whiles a friend of mine it does. Same engine same ECU same MAF.

Depends on the ECU. Some can be made MAFless, some can not. To make MAFless sw you need to have MAP sensor installed on the engine. Even then it is not only one switch to turn of the MAF.

You can also upgrade the original MAF to bigger one.

The thing is that I've an 1.6 HDi stage 3 mapped back in 2016 with 180HP and 360Nm with MAF ON and the standard plastic Intake. In 2017 I brought additional upgrades including a ported manifold and a 70mm free flow aluminium intake ( also relocated the MAF ) and started to experience downpower especially after 3200 revs. Also dynoed the car and I was down around 17 20 HP.

Than I started to log the car and I fond out that the MAF is getting maxed at 427Kg/h to an early 3100 rpm and stalls there and the IQ is getting lower from that point. Could these upgrades leans out the A/F ratio and now the ECU needs to be recalibrated to compensate for the change ?

With stock intake in 2016 and stock exhaust manifold should the tuner have gained the advantage of a larger MAF operational spectrum through the revs range whilst with this larger custom custom MAF is getting maxed out earlier..

PS. With MAF unplugged the power is there and the MAF readings reach around 600 Kg/h at 4500 revs.