OBLIGATORY WARNING: This may mess up your device. Don't do this.
It took me some time (even though in retrospect the mechanism is obvious...) - here's how you can make sure the enabled options of your Anritsu spectrum analyzer match the hardware, and enabled those that were accidentally disabled (like Ethernet).
First, you need to enable "Maintenance Mode". This can be done by sending the following command via SCPI:
MAINTE SIGANA,ONFor example, you can configure the communication interface to serial, and then send "MAINTE SIGANA,ON\n" with a null-modem cable. GPIB and Ethernet (if enabled) should work as well, but I haven't tested. There -is- a way to enable maintenance mode via a different mechanism (likely pressing a magic key combination? or an internal button?), but I haven't been able to figure out how. After that, press "LOCAL" so you get back control.
After that, enter the configuration (press "Config"), and then press "System". Without maintenance mode enabled first, you'd now be able to chose the "system" (i.e. select the software module when you press "TX Tester"/"Signal Analyzer"), but with maintenance mode, you get into a more advanced "Configuration" menu.
Here you can change the product configuration (Type/Model/Serial/Frequency spec), as well as reset the mechanical switch counters, sub-board module numbers, but most importantly, you can edit the enabled options.
I was able to enable option 002 ("Narrow Resolution Bandwidth" aka. FFT mode), and 009 ("Ethernet"). I tried switching option 046 ("Auto Power Recovery", i.e. non-functioning standby mode - which is actually a feature as most of the MS860xA have been used in automated test setups), but unfortunately it seems to be a hardware option (so I observed no change).
Some options are set to not be disabled ("NOf", "NOn"). Interestingly not all options have an entry , but you can just add them.
On my device, I can use the power sensor in the TX Tester, but not the power meter in the Spectrum analyzer (shift-option -> Power Meter says "you need the power meter option"). I've tried adding Opt 21 ("power meter" for Spectrum Analyzer; from MS2683A datasheet), and while it made me able to start the Power Meter, it's non-functioning. It's probably a different hardware option than the TX tester power sensor, unfortunately, or the calibration is way off.
Make sure you capture your set of enabled options first! Not sure if it's possible to backup the internal state to a CF card first.
I've tested this on version 4.8, the only one I had available (which was also pre-installed on my device). If someone has another version (I know Wuerstchenhund had one showing 4.12 in startup, but not sure if that's newer or older) I'm curious.
Have fun!