If your site is not that much complex then you can follow the Lift and Shift approach that is
1. You can install plane instance of sitecore 9.
2.Upgrade your solution to .net version 4.6.2 or higher.
3.Install necessary module like WFFM on plane instance of sitecore 9 .
4.Create the packages of sitecore 8 instances.
5. and install package on sitecore 9.