These changes may have caused the tones to shift from having at least three tones to five (or six in other dialects of Siamese). The devoicing of original voiced consonants made them merge with their voiceless counterparts. To maintain the distinguish, the tonal shift had to occur. For example, before the shift there were "va" and "fa". However, after the shift, "va" became "fa". Thus, now we have "fa" and "fa." To distinguish the difference, one (or both) of them had to undergo a tone shift: fa (mid tone) and fa (rising tone) etc. This phenomenon could explain how modern Thai has come to have three consonant classes (e.g. high f vs. low f---where the low f may have come from the ancient /v/).
Oh and have you ever wondered why ห is written before น to create a new sound: a high n? Well in the ancient time, these two sounds were actually a cluster, i.e. it was pronounced as /hn/. Linguists would go look at other languages that borrowed a Thai words beginning in hn in the ancient time, and see if in these languages still retain the hn sound. How did linguists come to conclude these changes? Well thankfully the Thai script was created based on Indic scriptures, so we can compare it with let's say the Devanagari script to see how the sounds differ since there are many Pali and Sanskrit loanwords in Thai.
For more info on the ancient pronunciation, read From Ancient Thai To Modern Dialects by J. Marvin Brown, and there are also a lot of great researches done by the late linguist William Gedney too which maybe in your local university's library. For those who are curious how the Thai r became h in many Tai dialects(and not the other way around since the creator used the same symbol to write a Pali word with an r and a Thai word with an r) such as Lao, Shan, and Northern Thai, these books might give you the answer.
Images above taken from here