Watch the Two-Beat & 4/4 online guitar lesson by John Goldsby from Upright Bass Handbook
To smoothly go from a "two" feel to a "four" feel, you need to feel the time. When you feel the time deeply, then all you need to do is place your notes right in the pocket. Then you can move from playing in two, to playing in four, with a smooth shift. Swing bass players like Walter Page, Jimmie Blanton, and Welmann Braud could ignite a band just by shifting from a two feeling into a four-four feeling.
When I play in two, I sometimes make a space for the backbeat on beats two and four. This helps the line swing. You want to play the notes on beats 1 and 3 long, but cut off on beats 2 and 4.