What you're observing is the main reason why FME doesn't automatically upgrade the transformers: to avoid any surprises in transformer behavior due to changes in the underlying transformer algorithm.

Concerning the mentioned settings :

• Connect Z mode: this is only relevant if you have 3D data (Z values). It is also available in FME 2016 and the default value "First Wins" is identical with FME 2018.
• Tolerance: This is a new setting in FME 2018. The default value of "None" is identical to the behavior in FME 2016. You can also read more about this setting here.
• Aggregate handling: in FME 2016 all aggregates were automatically deaggregated. In FME 2018 you can choose to send them to the <Rejected> port instead. If you leave the setting at the default "Deaggregate" it should behave as in FME 2016.

If you still get differences, there are two things I'd recommend:

1. Create a small sample workspace with data that reproduces the issue and send to Safe support so that they can determine if it's a regression or as expected. You can have both versions of the Dissolver transformer in the same workspace.
2. Do not upgrade the old Dissolver transformer, leave it as-is in your workspace to avoid any changes in the output.

I think the new Dessolver should generate the same result as the previous one, by default.

Make sure that the Tolerance parameter is set to 'None' (default).