Sadly I keep seeing a misinterpretation of this rule in numerous postings, most recently in Goatboys review of the Tyranid Vanguard dataslate.
He indicates that because Genestealers are part of a formation they are no longer Troops, this is 100% false I will explain why.
The rules for Formations clearly state;
A Formation presents a collection of two or more units that fight alongside one another in a particular way. When you choose an army, you can take a Formation as a special form of Detachment. Unless otherwise stated, you can take any number of Formations in your army, and each is considered to be a completely separate Detachment, regardless of how many units make it up. Each Formation will tell you what units you need to take and what, if any, options or restrictions apply to the units that make up that Formation. The Army List Entries for each unit in the Formation (the units’ profiles, points values, unit types, unit composition, special rules, battlefield role etc.) can either be found in the codex corresponding to the Faction on the datasheet, or elsewhere in the dataslate itself.
The key wording here is battlefield role, the first page of every 6th edition codex Army List defines battlefield role as HQ, Troops, Dedicated Transport, Elites, Fast Attack, and Heavy Support. The Tyranid Codex defines it thusly.
The Tyranids army list is split into five sections: HQ, troops, elites, fast attack and heavy support. All of the broods, monsters and unique creatures in the army are placed into one of these categories depending upon their role on the battlefield.
This wording is consistent thru all the 6th edition codices.
So by definition unless specified otherwise units that were troops in the corresponding codex remain troops. Keep in mind they are still treated as a separate detachment though example follows.
If we were talking about say a formation that included Chosen and you were playing using the rules for Black Legion, no they would not be troops since the Chosen in the formation are a separate detachment and the rule that changes them from Elites to Troops is only conferred to the Black Legion detachment.
I hope this helps to clarify this for any that may have been confused.