A puzzling article on Iran

There is something I definitely don't understand here. This article:
LA Times article on Iran potentially stalling with the uranium enrichment deal
states that Iran is further stalling the agreement to ship its low-enriched uranium to other countries for enrichment to highly enriched uranium.

Here is what I don't get. What Iran seems to be asking for is that they basically should be allowed to trade their low-enriched uranium for highly-enriched uranium fuel rods that are ready to go in their research reactor immediately. This doesn't seem too crazy to me. They seem to be saying they just want to be really sure they get what was promised to them up front, rather than allowing the international community to take forever in delivering the highly enriched uranium fuel rods (or perhaps never deliver them).

However, here is a quote from the article:

"Western officials have hinted that they would reject such a stipulation, worried it would allow Iran to quickly replenish its stockpile of enriched uranium to maintain the quantity that could allow it to fuel a nuclear bomb"

Does anyone see how this works? Why would arranging an exchange of low-enriched uranium for fuel rods somehow accelerate the time frame for Iran to get a nuclear bomb? Are they worried about a plutonium path to the bomb?

Curious whether anyone is able to follow what is going on here...