If traveling soon Krabi province will be in the rainy season which peaks in September-October. There should be plenty of available accommodation.
I would still research the likely places online in advance to avoid wasting time and effort traveling around looking for a suitable place. Higher-end hotels probably need to stick to their regulations and you are likely to get a lower rate online than the rack rate they would offer to walk-in customers. In a guesthouse you might get a lower rate as a walk-in by asking.
Quite easy to book anywhere on short notice. I wouldn't book much in advance, as you obviously having planned much or are familiar with the locations. May not like your destination when arriving, then stuck with your hotel. Book first nights only.
You're not going to recognize a lot of places. A mere 5 yrs, can drastically change a location, unless already maxed out on building area, when last time visited. Which guessing, isn't the case for you.
So many areas, have drastically changes since I first visited, and as stated, mere 5 years. Areas extremely popular and built up already, just a bit more so, won't be too noticeably change. Such as Pattaya or Patong over a 5 or even 10 year period.
Though others nearby or different areas may drastically be changed, such as Hua Hin over the last 10 years.
If logistically challenged, than not much market demand for building up, such as Prachuap Khiri Khan (municipality), and places like Ban Krut, or Thung Wua Lam over the last 5 to 10 years.