Presets Compatibility

Yes, you can convert it to an earlier version as long as you have an earlier version on your system.

After Effects CC 2018 (15.1.1) has the command Save > Save As > Save A Copy As CC (13).  CC(13) is CC 2014. CC 2014 can get you back to CC (12) and CC can get you back to CS 6 (11). If you are going that far back, I would just buy the script in the link below which creates a text file. It preserves most but not all of AE to go back to CS 3 (8). You still need the version to start to create your preset. 

Here is a page from Adobe for Opening After Effects projects from previous versions and saving back to previous versions

I would backdate your project and open it in an older version then resave the preset. That should work as long as you are not using any newer effects. You can download as earlier as CS 6 for After Effects. I actually keep CC 2014 on my system since it is the last one with the old engine. (Brainstormer and Pingpong)

I only checked the preset coming forward with CC 2014. That is the earliest version on my system. I assume it will work as far back as CS 6. 

If you create the preset in an older version of After Effects, it will work in the newer versions with no issues. If you create a preset in the current version of After Effects, it will not date back, instead, it will error out. This is the error message you get:

Hope that helps. (I tried it against the older engine in CC14 and the newer engine in CC15. The current version  is based on the CC15 engine)