sould rewrite to multioption:

openvpn-option tun-mtu 1500
openvpn-option fragment 1300
openvpn-option foo bar

@elbandi sure that I used as workaround.

The problem is if someone upgrades from 1.2.5 to 1.3.x this will break OpenVPN which is bad.

Crux probably passed this as a string of options to the command line or
split it into multiple options. We need to add a parser to split the
options by '--', hopefully this won't break any options which should
include -- in one line.

Just call the multiple options other name (like 'option'), and admin can modify they config if they want. no need a IM migrate script.
And leave the openvpn-option for raw option, if someone want to make a magic things in openvpn.
(like, in dhcp server, static mapping has option tagnode for "Host-specific settings" and static-mapping-parameters for raw hacking :)

@elbandi how should this approach fix the error on image upgrade?

If in crux this was just a option string passed to the openvpn process,
quoting or escaping could be used to pass a parameter with '--' to an
openvpn option. I need to have a good long look at how Crux did this and
have some real life examples. For example: openvpn-option '--mssfix --up
"/some/command --param1 --param2"' (I'm not sure if this would've worked
on Crux, the quoting is probably wrong, and the up option parameter too,
but my point remains that this could've been a valid option syntax)

We'd probably need to write a migrator script that was quoting and
escaping aware, because we can't just split by '--'.
I'll take a look this week after I get some free time.