Hey, thanks for playing it.
You can still unlock the previous ones using the new heads, for example:
you can unlock an upgrade that costs 3 heads from the first zombie, using 3 heads from the second third, or fourth zombie.
It always accepts a higher tier head, :)
Viewing post in Idle Game with no Name (Zombeez Game Jam) comments
sorry for not mentioning it ingame.
you can also try this version - https://vkaike2.itch.io/zombeez-game-jam-post-jam
is an updated version, and it contains a better behavior, on this new version it works like this, let's say that you have.
1 - head from zombie 01
2 - head from zombie 02
and want to unlock something that costs 3 heads from Zombie 01.
it will take 1 from the first zombie and 2 from the second and unlock for you.
on the current version that you played it is like this:
1 - head from zombie 01
2 - head from zombie 02
and want to unlock something that costs 3 heads from Zombie 01.
it will not allow you to unlock it because it will require 3 heads from zombie 01 or higher, so it will require 3 heads from zombie 02.
I hope that I could explain it properly, thank you for playing it, it means a lot to me.