Obviously when you upgrade, the connection WILL drop since the device has to reboot.
And then it remains to be seen if nothing major has changed between 6.25 (!!?? That’s from january 2015 ??!! ) and 6.49.18 with respect to IPSEC.
Do you have the possibility to add a second VPN next to IPSEC, as fallback ?
Which then brings the question … if this is only for a few months and it’s such a possible risk with the device being remote, why upgrade ?
If it works, let it be.