There is no evidence from ancient sources that any helots were made citizens in the aftermath of Plataea. Also, there is very strong circumstantial evidence which suggests that it didn't happen. The earliest likely date for helots becoming citizens is the late 3rd century, at least 250 years after the Battle of Plataea.
There is also no evidence that any helots were freed in the aftermath of the Battle of Plataea, but it is not inconceivable that a small number were.
The role played by the helots at the Battle of Plataea (479 BC) - and how many of them there were - is disputed. The only source is Herodotus who says there were 7 helots for each of the 5,000 Spartiates (citizens):
this would certainly be the
largest number of Helots ever known to have left Lakonia. In fact to
many scholars it has seemed implausibly high. Clearly it was not
demanded on strictly military grounds, although Welwei (1974, 120–4)
has properly stressed the supply problem of this campaign and
suggested that Helots were used to solve it.
Source: Paul Cartledge, Sparta and Lakonia
Thus, many helots may not even have fought but, for those that did, there is no evidence in ancient sources that any were made citizens.
Despite the Spartan citizen population declining steadily over the years from around the late to mid 5th century BC, only those born to a Spartiate father and a Spartiate mother could be citizens. Even the sons of Spartan citizens with helot women (mothakes) could not become citizens.
Also at Plataea were 5,000 perioeci hoplites. These were free non-citizens who had no political say in Sparta and were obligated to provide soldiers when called upon. They were of a higher social class than the helots, and yet there is no evidence that any of them were made citizens for the military service they rendered at Plataea. If the perioeci and mothakes could not gain Spartan citizenship, it is highly improbable that helots could.
Although the shortage of citizens became increasingly critical during the 4th and 3rd centuries, proposals to increase the citizen body by granting citizenship to some of those outside the elite were rejected until Cleomenes III (235 - 222 BC) forced through much needed reforms after an earlier King, Agis IV (245 - 241 BC), had failed.
We cannot rule out the possibility that some helots were freed in the aftermath of Plataea, but there is no mention of this happening in ancient sources and it does not appear to have been the practice at the time of Plataea. If any were freed, they would most likely have become perioeci and been given land on the frontiers of Lakonia or other territory controlled by Sparta.
The practice of freeing helots for military service rendered does not seem to have happened until around the 420s (see Neodamodes, and also those who fought with Brasidas during the Peloponnesian War). The JSTOR article On Messenian and Laconian Helots in the Fifth Century B.C. goes into some detail on this by examing the ancient sources.
This change in 'policy' was likely due to the apparently heavy losses Sparta suffered in the earthquake of 464 BC, and the casualties suffered during the Peloponnesian War. Note how concerned the Spartans were at the possibility of losing just 120 citizens captured after the Battle of Sphacteria (425 BC). Freeing helots was one way of tackling the problem of declining manpower.
Based on the available evidence, it is most likely that the vast majority of the helots simply returned to their former lives (and families, as well as the land to which they were tied) after the battle.
Anton Powell (ed), A Companion to Sparta
Nabis of Sparta and the Helots