Do those changes drastically change the way a game plays out? Esp. interms of pacing and overall strategy. Or does it just add some new cool features but nothing game altering.
One of each.
Entrenchment really changed the overall pacing and strategy with its starbases. It opened up a whole wealth of new approaches and tactics. Definitely a superb improvement to the game. Aside from playing multiplayer with friends who only have the base game, I've never looked back from here.
Diplomacy definitely is the latter; it added some cool new features that only altered the game subtly. It's definitely the weaker of the two expansions, but not bad in any way. In that sense, there's really no reason to go back to Entrenchment from Diplomacy.
I suspect that with Rebellion it will be the same, and once I jump in I won't look back. As was suggested in the other thread, the real question is what value you'll get out of the game between now and the release of Rebellion.
Say, more turtling with Entrenchemnt
While you can turtle against the AI, playing pure defense against a human player will get you nowhere. Veteran players use starbases in a passive-aggressive manner, allowing them to go on the offensive and defeat enemy forces with superior numbers and firepower.