Can I transform Westvale Abbey during either assign attackers or assign blockers phase
You can transform Westvale Abbey at several points of the Combat phase and potentially use Ormendahl as an attacker or blocker.
The Abbey's abilities other than its mana ability are normal activated abilities that require you to have priority.
There are 5 steps to combat:

506.1. The combat phase has five steps, which proceed in order: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat.

During beginning of combat, everyone gets priority in APNAP order and you can transform Westvale Abbey. This is your last chance to transform if you want Ormendahl as an attacker this turn, and the other players will still get the opportunity to play instant spells and abilities after the Abbey has transformed.


Beginning of Combat Step

507.2. Second, the active player gets priority.

After everyone has passed priority, attackers are declared. If you are the attacking player, you can declare Ormendahl as an attacking creature if you controlled the permanent (Abbey/Ormendahl) uninterrupted since the  beginning of your turn, i.e. it has no summoning sickness. Directly afterwards, players again have the opportunity to cast spells and activate abilities. If you want Ormendahl as a blocker, now is your last chance to transform the Abbey.


Declare Attackers Step

508.1. First, the active player declares attackers. [..]
508.2. Second, the active player gets priority. [..]

Note that this means that as soon as the declare attackers step has started, anything that isn't already a creature cannot be declared to attack, and it's too late to transform the Abbey.
Next, blockers are declared (including another round of priority). Again, anything that isn't already a creature can't be declared a blocker.


Declare Blockers Step

509.1. First, the defending player declares blockers.
509.4. Fourth, the active player gets priority.

Finally, damage is dealt and combat ends.