Can you re-stain over varathane?

The way stains work is by pigment transfer. What this means is that the pigment in the stain (the colour) is mixed up in a carrier (the liquid) and as it is applied to the wood these pigments lodge into the pores and fibers of the wood. After the carrier evaporates the pigment is left behind, colouring the wood.

Because your bed has had the wood sealed, there is nothing for the pigment to stick to, so simply going over the existing finish isn’t an option. You do have a couple of ways you could go however:

1. Strip the existing finishes (I’d recommend a quality chemical stripper), finish sand the piece and re-apply the stain of your choice, then topcoat again. Or:

2. If you have spray equipment, you can order tinted topcoats from most professional paint and finishing stores. Because the colour is right in the topcoat, the more coats you apply, the darker the colour becomes. If possible, bring a sample of the colour you have, as well as the can of the topcoat you used with you into the store. They can then ensure the new topcoat and the old will be compatible.

Good luck with the refinish!