I'd strongly suggest to upgrade in multiple steps - due to different framework versions needed.

first upgrad to DNN 3.1.1, next to 4.0.3 (and ASP.Net 2.0).

continue to upgrade via 4.3.7, 4.6.2 to current version 4.9.4

I moved all my clients from V2 thro to V4. I did the following after a bit of playing around and getting errors

V2 to V3.1.0

V3.1.0 to v3.3.5

V3.3.5 to V3.3.7

this gave no errors - just about every other way gave some error in the sql scripts.

from there I created a new 4.9.3 site for each client in a sub directory and transferred the content via the export portal template system - changing the 3 to a 4 in the template header after testing then copied the new V4 site over the old site.. This depends on what modules you use in the portal. If you have large number of users then you will lose them most likely in the update fromV2 to V3 ( user system had a major change between versions)

>At least then I wouldn't have to worry about losing time for a bad upgrade along the way.

I do all updates on a copy of the client site on the offline in house test bed then once it works then do it live  or do a create via the template system on a new site