This is a marker telling you it's time to start limiting your morning nap.
The transition from 2-nap days to 1-nap days happens by the 2nd nap of the day being dropped and the first (only) nap getting later and later. So your morning nap will become your lunchtime nap, and you'll drop the one you now call the afternoon nap.
So how to do that? Two pronged attack, gradually changing over time.
Set a "wake no later than" time for your morning nap and wake baby up at that time, if not already awake.
Set the time such that it will still allow for an afternoon nap (as your transition to a single nap). I would suggest 10.30am from the timings you give.
Start doing naps by the clock. Say naptime of 9.30 and wake at 10.30. Don't put baby down earlier and don't let baby sleep longer.
When on 2-nap days the recommended awake time is 2h-3h-4h, so a 10.30am wake from a morning nap would suggest a 1.30pm 2nd nap (3h awake time). If you think 2pm would work better though, stick with that.
Likewise set a wake-by time for the 2nd nap. Allowing a 4h awake time to get time, a 2pm-3pm nap would mean 7pm bedtime. So wake baby by 3pm at the latest.
For this to work you also need to wake baby in the morning at a consistent time, as you would if setting a daily alarm.
Then progress from this point by making morning nap later and shorter. You could move a long the lines of:
- 7am wake. 9.30-10.30 nap, 2-3 nap, 7pm bed
- 7am wake, 9.15-10.30 nap, 2-3 nap, 7pm bed
- 7am wake, 10-10.30 nap, 1.30-3 nap, 7pm bed
- 7am wake, busy morning, 1pm-3pm nap, 7pm bed
That final transition to 1-nap days need not be an all-or-nothibg change. It might be that you have a single, solitary day where you stay busy in the morning and just have one nap. Then the rest of the week you have normal 2-nap days.
The next week maybe you have 2 days of just 1 nap. Probably in the form of a 1-nap day followed by 3 or 4 2-nap days. Then maybe two consecutive days of 1-nap followed by a day or two of 2-naps then back to 1-nap days.
Do you see what I mean? It's a gradual transition rather than a sudden change. The same with the changes in timings of the 2 naps. You might have a step forward and two steps back. But then a jump forward again until you get used to the new stage. And so on.