While buffy juvenile Semipalmated's do exist, they don't approach the overall buffiness of the entire head and breast of a fresh juvenile Baird's. The scaliness and long primary projection also help the ID.
Here's another semi for comparison:
It has a shorter bill, primaries that stop at the end of the tail and less overall scaliness.
There was an adult Semi, I think the first one I've seen recently.
It's still in mostly worn breeding plumage with old wing coverts and a lot of worn scapulars, though there're a few gray winter scapulars starting to fill in.
An adult Semi Plover was in similar condition,
a lot of worn wing coverts, but brighter legs and bill base than the fresh juvenile...