Wait, you can't mix and match parts now? That's too bad really.
My wife and I have 4 bikes with Ultegra Di2. Two with 10 speed and two with 11. Both of the 11 speed setups are using the "10 speed" shift/brake levers, as we already had them and saw no reason to upgrade those.
Good to know the latest firmware might break the setup? We have multishift enabled already, so no real reason to update further.
You're still fine, the Shift Units (ST / SW) are essentially "dumb" devices, they don't know and don't care how many gears you have. All switch units, according to Shimano are interchangeable (see E-tube compatibility chart on Shimano's site). Switch units include: ST-9070, ST-6870, ST-9071, ST-6871, ST-6770, ST-R785, ST-S705, SW-9071, SW-E6000, SW-R671, SW-R600, SW-S705, SW-R610 and SW-M9050 (you may choose a maximum of 6 units from this group). All switch units work with 11-speed or 10-speed.
The only "old" part that is not compatible in an upgrade is Junction A SM-EW67-A-E if you decide to upgrade to Battery SM-BTR2 (Internal), then you also need to upgrade Junction A to either SM-EW90-A (3 port), SM-EW90-B (5 port) or SC-M9050 (combo Display/Junction A), all because you will need a charging port. Please note that Junction A SM-EW90-A / B or SC-M9050 will work with either the external SM-BMR1 / SM-BMR2 or the internal SM-BTR2.
A second note, if you "upgrade" Junction A to any of the newer three units you can also directly connect them to the E-tube software and do any upgrades without the need for a SM-PCE1 since you will already have the SM-BCR2 to charge the internal battery. The only current limitation between the SM-PCE1 and SM-BCR2 is that the SM-PCE1 allows you to also do diagnostics on your connected E-tube components.
Also note that the SM-EWW01 Wireless unit also doesn't care if it is hooked into a 10-speed or 11-speed system.