Hi - you don't need to worry too much about following the "plan" exactly - it is a guide just as any other "daily schedule" that Tracy includes in the books.
if you think your lo is ready (and not just because of her age) for the transition do it as slowly as you need - if increasing A time by 15 min is too much at this stage try 5 min increments. STILL FOLLOW YOUR BABY'S CUES. if 5 min goes well try another 5 min increase 2-3 days later. you will get there in the end but the key things are to still ensure that your lo is getting to sleep when needed and that you still do E/A/S/Y not to worry about exact times...
when olivia was 4 mo and could do A time of 1 - 1 1/4 hrs we started to transition from 3 to 3 1/2 hr EASY and at 5 months we made it to 1hr 45 - 2hrs awake time and 4 hr EASY started to work for us, until then i did not push her, just tried to extend A time when i could see she was able to comfortably stay up longer and still go down easily for naps. HTH go at your own pace
PS - if she is teething and taking less food she might struggle with extending A time right now if she is not getting enough food to keep her going - you could try offering her a top up after a 15 min break and see if she wants a little more (that is if her intake has significantly dropped off and you are worried)