I want to upgrade my Mikrotik OS ver 2.9.51 to 3.27. But i want to know, whether the configuration system after upgrade will be saved? I need to save configuration vlan’s and network interface’s at minimum.
Configuration will be saved. But if you are worried then you can always make backup or do /export before upgrade.
I have only remote connection on this server, if configuration vlan’s and network wil be lost, i can’t connect to it.
Guaranteed save configuration vlan’s and network interface’s after upgrade?
I have done a lot of upgrades. Probability that you would not be able to fix it if something is different is very little. Just make sure the new .npk packages that you upload are the whole file, and not some incomplette download! Very important!!!
Just once or twice, I saw an IP address of an interface getting disabled or assigned to an unknown interface… so if you have connectivity to an adjacent router that has a layer2 connection to your MT you want to upgrade, - just keep mac wibox and mac telnet services turned on (tools menu … ) so that you could at least mac-telnet from the neighboring router.
And think when to upgrade - when will you have time and opportunity to go to the router and fix it, if connectivity is lost? Late at night? On a certain date? Early morning?
What if NAND/Flash fails? You should be ready to reflash viq RS232 or replace the unit so that clients traffic could continue thorught those link its on.
Probabilty of a failure when you upload the whole .npk are really slim. I can bet 10$ that it will work just fine. 
P.S. oh and lets not forget that you need to upload only the NPKs that you are using and you need to check for free space before you do the upgrade!!
Sometimes I uninstall The Dude - takes a lot of space, uninstall unneeded packages, reboot, twice if I have to, and then upload the carefully selected new .npks. I double-check, I tripplce check if all npks are in place and uploaded OK and then I reboot for them to be auto-installed…