If your damage was at 302 or higher before upgrading then there was no point in trying to upgrade it. The upgrades are for people who have the pre-patch version whos max damage is 300.
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It is also said in the Xur update, that you can actually keep doing the process but you will not upgrade it anymore than what you have already. Meaning if it was 302, then he resets it and its still 302 but wasting the Glimmer and Exotic Shard in the process.