Hi, I hope we can get some tips here or a straight forward answer 
. We got some RB951Ui-2HnD with a factory FW and RouterOS v6.43.2. For a particular reason we need to downgrade the RouterOS to 6.42.6 (or some other 6.42.x version). We tried everything we knew so far but it just doesn’t allow a downgrade. Does anyone know if it is at all possible to downgrade RouterOS to a version lower than the factory set one, and if yes - HOW?? Thanks in advance for any help!
The idea behing lowest supported version is that sometimes there may be small hardware changes, requiring also software support. If you’d downgrade to older version, it wouldn’t work correctly. So now you can’t.
Regular upgrade/downgrade is done by currently installed system, and if it checks minimum version, I don’t think you can get around that.
Second way how to install system is Netinstall. I didn’t test if it checks for minimum version too. If not, that would be a way. If it does, then some older version doesn’t. But in any case, I don’t think it’s good idea, you might end up with brick instead of router.
Maybe if you post why you need to downgrade, we can be able to solve it without need to downgrade.
@Jotne: I will gladly explain our problem. This is a problem that’s been causing us some concern and it shouldn’t be rocket science, but we could not solve it so far. Here is the explanation:
We have a hotspot system using almost exclusively RB951Ui-2HnD routers. We have a login page hosted on our (radius) server behind a https:// address to give our users added security. With a v6.42.6 and some other v6.42.x when a user selects the hotspot WiFi network, the Captive Assistant in iOS and Android do their usual checks (with “Success”) and after realizing that it’s a hotspot network, they reroute the users to our https://hotspot.mydomain.com URL. There, the login page is displayed and the user is able to complete the login process.
With later versions this is not working! The same thing happens, user selects our hotspot SSID, the Captive Assistant kicks in, the user is redirected to https://hotspot.mydomain.com URL but the page is blank (light gray and no content of the page is displayed). Also there is no security warning or similar. There is just an empty light gray page in the limited browser of the CNA. This makes troubleshooting difficult. Same thing happens in a regular Web browser on the phone. When I enter manually in the browser the http://hotspot.mydomain.com, the http:// page is displayed.
Obviously it’s to do with https but we don’t understand why the browser does not open the https:// page anymore with newer versions than 6.42.6 but http:// works. We really want and need to have our login page behind a https:// and i think this is standard today so I cannot believe that Mikrotik suddenly doesn’t support this. Please help if you have had experience with this before. We are really stuck! I can provide any traces/screenshots etc that you might need to give you further clues. THANK YOU!
Have you looked at this thread?
http://forum.mikrotik.com/t/hotspot-and-https-what-solutions/117017/1