DETAILS June 6, 2020: I was able to find a workaround for the problem referenced by the original poster of not being able to change the icon type in a LibreOffice Calc Chart. However, it required downgrading or upgrading to LibreOffice (Version 184.108.40.206) or up to the LibreOffice 7 beta version.
Claim: The problem of NOT being able to change the icons in a Calc chart is almost certainly isolated to LibreOffice 6.
Evidence Backing Claim I tested the ability to change the icon of a Calc chart in three LibreOffice versions: 220.127.116.11, 6.4, and 7.0 beta. I was able to successfully change the icons in LibreOffice 18.104.22.168 and the LibreOffice 7.0 beta version.
Logic connecting evidence to claim. Because I was able to successfully change the icon in 22.214.171.124 and 7 beta, this strongly suggests the problem is isolated to version 6.X. This is further supported by the facted that SeijiSensei also commented that he had similar problems on his Linux installation of 6.X, which bolsters the support that it’s the version and reduces the possibility of this being a combo error (OS & version). I assume that I installed the different versions (126.96.36.199, 6.4, and 7 beta) without error and that having all three on the same system would not cause any unexpected behavior. Of note, I only had one version open at a time when testing.
Short-term Solution: Use 188.8.131.52 if it’s that important to switch icons.
Long-term Solution: It appears 7.X will fix the problem, but the beta version isn’t stable enough for daily use. As an example, it appears 7 beta is not able to load files from disks as I had to manually recreate the data in Calc to run the test. The “File-> Open” workflow would spin without any resolution.
MacBook Pro (13-inch, Mid 2012)
OS: 10.15.4 (19E287)