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I don't think this is the right fix, as from target version we have just the same information. Each function corresponds to one opset pair.

The core issue here is the model supplied is opset 17, which is not supported by this converter. We can add support for that.

This is might be part of the problem, but I think the underlying problem is still how the converters are selected independent of opset version:
Take the Acos operation as an example:
If we have an model with onnx opset 19 (which is supported by the version converter already), then the Acos version will be v7.
If we now upgrade the model to onnx opset 22, then this loop will loop from 19 to 21:

If we now have our conversion function registered like this
@register("Acos", node_version=7, up_conversion=True)
then the version converter loop will never find it, because we start at 19. This is wrong, because operator version 7 belongs to model version 7-21. We have to directly upgrade from operator version 7 to operator version 22.
So the upgrade has to only trigger if the version to which we want to upgrade is 22. Because all other upgrades are noops as operator version 7 is still the newest one.

In my opinion, the easiest way to achieve this behavior is not to look at the source version, but the target version. Then we get for an up_conversion registered for opset 22 automatically support for all model source opsets from 7-21 (or 18-21 if the old opset bound for the converter is kept).

I tested the change by implementing an version converter for ReduceLogSumExp in onnx-passes: iksnagreb/onnx-passes#4

An alternative approach would probably be to register the nodes not based on operator version, but based on the last model opset, which they support before an upgrade. So for the example Acos Op, this would then be @register("Acos", needs_upgrade_after_opset=21, up_conversion=True) or something along this line...

Thanks for your analysis! Will take a closer look tomorrow