Development Team members moves to Proxy PO Role - What changes should be made.

You've described a scenario that is separate to Scrum. So rather than answer directly, it may be good to explore the subject.

What is meant by Proxy Product Owner?

Is this person accountable for the value delivered?

If they're not the actual owner of the product, could it be that their contribution is more akin to that of a member of the Development Team who contributes in a specialist way?

One of the Development Team members has been move to a Proxy Product Owner role. This individual no longer develops.

My question...Should they still take part in Poker Planning to determine the size of the Stories in the Backlog or is there now a conflict of interest?

How much work is involved in proxying? I'd suggest that it introduces management overheads in crafting a "Done" increment, which ought to be transparent, and accounted for.

It's conceivable that the Product Owner while remaining accountable for the Product Backlog, may have the Development Team do some or all of the work. However, I don't think the intention of that is what you describe, especially if the Development Team member has to fully stop carrying out the other work associated with being a Development Team member. There seems to be something going on here that I would want to understand.

However, this strange structure doesn't mean I can't look at the question. In this situation, since the "proxy Product Owner" is no longer carrying out the role associated with a Development Team member, I would no longer consider them to be on the Development Team. Since the Development Team is responsible for all estimates, I would not think that this person would be involved in determining the work's size. This is consistent with the generally accepted idea that the person best suited to estimate the task is the person (or people) who would be carrying it out. However, this doesn't mean that the "proxy Product Owner" can't or shouldn't be involved in the discussions. Rather, the people who actually make up the Development Team need to create and stand by their estimates.

One of the Development Team members has been move to a Proxy Product Owner role. This individual no longer develops.

That statement is the key for me.  If this individiaul no longer develops then they are no longer part of the Development Team.  As @Thomas Owens points out, only the individuals doing the work are responsible for the estimates.

This comes from the Scrum Guide's section describing the Product Backlog

The Development Team is responsible for all estimates. The Product Owner may influence the Development Team by helping it understand and select trade-offs, but the people who will perform the work make the final estimate.

So the "proxy Product Owner" can be involved but only for the purpose of helping the team understand and select trade-offs.

One of the Development Team members has been move to a Proxy Product Owner role.

The way this is worded intrigues me. Specifically the passive voice.

I keep trying to imagine what would cause someone to have been moved to this role, rather than to have moved themself.

How self-organizing are this team?