Can you please tell me your plan? Marcus will go to Udon on 26th. How's about yourself? I originally book the following flight for you - leaving for Udon with evening flight on 25th Mar and returning on 28th Mar. if you don't have any plans in Bangkok on 25th, why don't you come with Ken and Richard in the morning. They will get on a morning flight ( 0725 - 0830 ) on 25th. Or you still like to stick on the existing itinerary.
Have you decided about your returning date? If you want to attend a workshop, you have to come back on 28th Mar. Or you come back with Ken 7 Marcus on 27th Mar after the field trip.