Yes, the 757 is doing the 13:10 LGW-RIX e.g. on Fridays, see their website. I've been on it in May. Nice flight, wide seat with lots of legroom. My connecting flight RIX-VNO actually was also a 757, the second one.
Check e.g.
Transport, The Baltic Course, they have announced so many new routes, I don't see why they should change the plan on the Q400s.
Or you can ask them about this, as they are on Facebook
airBaltic | Facebook 
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