Yes, frequency is not changed
Nothing in the discovery itself states otherwise, which means we have to fall back to the general rules for Afflictions and specifically Poison to look for an explicit link between type and onset/frequency.
In the affliction rules, the type section does not mention frequency and frequency does not mention type. As @HeyICanChan pointed out, the poison rules are more specific.
Contact: [...] Contact poisons usually have an onset time of 1 minute and a frequency of 1 minute.
Ingested: These poisons [...] usually have an onset time of 10 minutes and a frequency of 1 minute.
Inhaled: These poisons [...] do not usually have an onset time. [...]
Injury: These poisons [...] do not usually have an onset time and have a frequency of 1 round.
These onset and frequency values are incomplete, as there is no typical frequency given for the inhaled poisons (the text does mention requiring a save every round, but that is only for the initial contraction). They are also not rigorous, as indicated by the use of the word "usually".
An attentive reader might have noticed the irregularity in the text for injury poisons, which actually explicitly state that injury poisons "have a frequency of 1 round". Grammatically, this is because the negation for the onset part was moved in front of the "usually"*, effectively changing the logic by turning "usually (A and B)" into "(usually A) and B".
However, the existence of several injury poisons which don't adhere to this rule, such as Sea Urchin Venom (1/min) or Woundweal (1/day) proves that this rule is just as weak as the other ones.
Overall, I'd say the frequencies of typical poison types are more what you'd call guidelines, and in the absence of anything that would explicitly change the onset and frequency of a poison converted via this Alchemist discovery, it stays the same. Do note that while this makes turning something into an injury poison a bad idea, you can create some nasty spiked drinks.
* Pure speculation on my part: This mistake might have been created by copy-pasting the first part from inhaled and appending the rest, or even during proof-reading of the rules. Someone changed "usually do not have" into "do not usually have" (which is formally correct), without taking the context into account.