If someone signs up for a credit card and the offer soon-after increases, some banks will match the better offer and increase your bonus upon request. Chase has been known to often do so, but not always. The easiest way to find out is by Secure Messaging and requesting a match.
We’ve seen recent signup bonus increases on the Chase Sapphire Preferred card and on many of the Chase United personal and business cards.
We’ve already seen a report of Chase being willing to match the 100,000 Sapphire Preferred signup offer. The report indicates they are matching signups since March 13th. Another report says March 12th. (Apparently, though it’s only available for now in branches, they are already matching the offer if you signed up recently.) Multiple reports in the comments here confirm that Chase is matching the 100k Sapphire Preferred bonus. (1, 2)
Conversely, the reports we’ve seen on the United cards are not looking good; thus far they seem to be denying to match the new offer, even for those who signed up recently. (1, 2, 3, 4, 5, 6)