You create your new LAG (2x10G) and you add it to the ISC VLAN (without cabling the ports) and you configure the necessary vlans on it too.
The idea will be to unplug the 40G links and plug the 10G links. But for some time, you have no ISC.
If everything is dual-homed :
> If you are running 15.5+, you can configure the alternate health-check path feature to handle automatically this situation.
> or you shut the MLAG port yourself on one MLAG peer.
You remove the 40G links, then plug the 10G links. You remove the 40G LAG from the ISC vlan.
And alternate way to do it, that is not validated by our SQA and *should* work, is to use SRP feature. Once again you create a new LAG for your 10G links, add it to ISC VLAN and add the relevant vlans to it. You configure this port as the backup port fo the 40G LAG in SRP (Software Redundant Port). You plug the 10G links. Once it's ok, you unplug the 40G links.