OK, this is good practice.....
Get your pan back under there.
Pull the drain plug out and dump the new gear oil out.
Put the bottom 'drain' plug back in.
Remove the top 'fill' plug and put in 3/4ths cup (smidge over 6 ounces) of your gear oil.
If it is running out the top hole - you did not let the old all drain out.
If you did drain it all and it still ran out the top hole.....I am flummoxed!
Many scoots just fill to the top 'fill' hole - most KYMCOs seem to require an exact amount when changing the gear oil....and a different amount when re-assembling the gear box. Re-assembling amount is more than the simple change amount. Obviously some little bit stays in there when you do a simple 'change' drain.
When I change my gear oil using the correct specified amount (6+ ounces, like yours) the level comes to just a little ways below the top hole threads ( I used a bent wire to reach in there to see where the level was)
Bottom line ?
If you put in 6 ounces after a good draining....let the excess come out - go have a donut and a coffee while waiting.
You should be OK then to put the fill plug back in.