So it sounds like your original wheel was a 6 speed freewheel. And you were using a 6spd thumb shifter.
Now replaced with an 8spd cassete wheel, and switched the thumbshifter into friction mode.
Is that the correct assesment?
So the thing is, the spacing between the cogs in a 6spd is going to be different from with an 8spd.
So the position of the 'clicks' in your old shifter will not match.
In theory, you could dissasemble the 8spd cassete by drilling out the rivits holding the cogs together.
Then choose 2 cogs to throw away (probably the largest 2 will be simplest).
Replace the plastic spacers between the cogs with wider ones so the remaining 6 cogs are at old style 6spd spacing.
then the old shifter can be used.
Thats a lot of work though, the hardest part will be getting spacers the right size. 6spd cassettes(non-freewheel) with removble cogs and spacers did exist, but are now obsolete, so finding spacers from one will be extrememly difficult. You'll also probably need to finagle something for the spacing of the smallest lockring cog since it uses an iintegrated spacer not the plastic ones.
So yeah, it is possible, but will be cheaper and less work to just upgrade your shifter to an 8spd one.