Till the time you can reach your router and your router can reach your tftp server YES you can.
But once IOS is upgraded and you have already set the bootpath correctly you have to reload the router which will let you loose the connectovity.
The risk involved is that after upgrade and reload if your router does not come up you have to console into the router and perform the troubleshooting. So if you can maintain a console connection before upgrade you are on safer side.
Also take the backup of your config before upgrade.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...