I'm currently running "DD-WRT v3.0-r30796 std" on a Linksys WRT1200AC REV2. I want to upgrade to the latest version 19.07.2. Can i do a direct upgrade through the web interface or do I need to use an intermediate firmware version first.
Welcome to the forums and (soon) to OpenWrt!
From DD-WRT, flash back to the stock Linksys firmware first. Then you can download and install an image of OpenWrt.
Most routers have two different images available: factory and sysupgrade. You will always use factory when you're coming from the stock firmware, and sysupgrade is used when you are already using OpenWrt.
Here is the OpenWrt Table of Hardware entry for your router.