Replacing any speaker (example: 4 ohm) with a new speaker with a higher impedance (ex: 8 ohm) should not cause any electrical damage to the amplifier. As an extreme case, if you removed the speaker altogether and left the wires dangling, you have a "speaker" with infinite impedance. The amplifier would have no trouble driving an open circuit.

Of course, installing a speaker with lower impedance is asking for a cooked amp, but that's not what I understand the situation to be in this case.

None of this changes the fact that the replacement speakers might sound worse, or cause the overall balance to be screwed up, because the higher impedance will mean that less power will be delivered to the new speakers than the old ones, potentially causing a strange-sounding system. Or not.

I say... go for it and let us know how it sounds. I wouldn't worry about cooking the amp.