We have updated ChartJS in the past few months to keep it up to date with the Modeler versions. As Ronald says, if you are using Simplecharts, please remove that one and replace it with ChartJS. If it's ChartJS, please make sure you download the latest version from the AppStore. Our tests succeed in the latest versions.
The Mendix Modeler version number is not very informative. It says 6.1, that means 6.1 and UP in this case (and I've made sure it still works in 5.18 and up as well)
If you still have issues with ChartJS after updating, please file a ticket at Support (ChartJS has extended support), or if you don't have an SLA, file an issue on Github
I've had no issues using the ChartJS widgets in projects created in Mendix 6.1+
Are you sure that the source of your problems is the ChartJS? Could you elaborate on the issues you have? Because it was the simple chart widget that would need to be converted because that one no longer works in Mx6. And as the rest I have no issues what so ever with the ChartJS widget that we used to convert to old simple charts.