Unless you're tripping your 100 amp main, why would you think an upgrade would
stop the lights from dimming?

You just might have a poorly-balanced panel!

15 years ago my Uncle wanted to upgrade a 100a split-buss Federal Pacific panel
to 200a because he tripped the "lighting main" (60a) a couple of times in the
summer when the a/c was on.

Over the lighting main, the 40a range and 30a dryer. Under the lighting main
was the pool filter, a 30a electric water heater, a 220v 18,000 btu a/c and the
rest of the typical branch circuits.

His usage wasn't demanding more than 100a, but one leg of the 60a lighting main
would demand more than 60a when conditions were right, specifically the pool
filter running, the A/C running, and the water heater running simultaniously.

By simply changing the panel to a standard 100a main panel his problem was
solved and remains trouble free to this day.

Lights will always momentarily dim when a heavy load such as a pool filter or
vaccuum or washing maching switches on or changes gears, and especially when an
older window shaker tries to chime on...