You’ve not provided release details; which are what i’d consider first. Are you intending to re-install the same release? or another?
Firstly I’d boot live media (ie. the TRY mode on Lubuntu livecd) and ensure you have a good backup of your data… its easy to make a mistake, and you need to be confident that you can restore your data should something go wrong.
As for non-destructive re-install, I’ll provide the following links
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Testing Checklist - understanding the testcases where search for the “Install using existing partition” type of install. This was not intended to be read by end-users though, but Quality Assurance testers, but I’d hope still makes some sense.
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system installation - How to reinstall Ubuntu in the easiest way? - Ask Ubuntu covers the same material, but hopefully may help you better understand what to do. I wrote that answer (on an old question) as I was asked to write it (the answer there wasn’t specific to Lubuntu, but it works with Lubuntu).
I use non-destructive re-install somewhat often (>30 times per year), but I’m very familiar with it. I’ve love to say it works perfectly all the time, alas I cannot, as a clean install (followed by restore of data) does tend to have a higher chance of success - but I love it (and have success >95% of the time). If you have issues with 3rd party software (auto-reinstalling), I’d ignore that & manually correct that post-install. Losing data using it I’ve never experienced.
Please note: We don’t use 3rd party software in our Quality Assurance testing, and you’ll note I don’t talk about that 3rd party software in either link I provided. I’ll suggest you assume no 3rd party software will survive… as the non-destructive install is intended for Ubuntu repository software only. Yes I’ve had 3rd party software re-install automatically, and can usually predict what will and won’t be coped with, but that detail gets very complicated & can be release specific, thus I’d not wish to advise in that regard.