Using things from previous packet, okay or no?

We are going to start playtesting the newest packet tomorrow, and today my players are reworking their characters to reflect the changes. I have a question regarding the specialities as my players really liked some of the ones they originally had - specifically the Guardian, Archer, and Two-Weapon Fighting. 

Is there any reason why I shouldn't continue to allow those specialities while running the newest material? Will it break the game mechanically? I know that in the case of Two-Weapon Fighting they added it in as a normal combat option, but I guess I see that as trying to do it without training and the training would be the specialty.

Thanks for any input.  
First and foremost it's your game and if you wish to continue using these options then by all means do so. However, if your intent is to playtest and provide unbiased feedback on the options presented then perhaps this is not a good idea. It is almost guaranteed that adding any house rule or older options to the new packet will have consequences that may or may not be obvious during gameplay. These consequences could bias any interaction in either a positive or negative way. When presenting feedback on the options presented you may not be able to distinguish how adding these rules influenced your perception of the packet and could therefore give biased feedback based on how your rules interacted with the playtest. So really it's completely up to you and your group. If you wish to simply play the game and enjoy yourselves then be all means modify the game in any way you see fit to add fun and enjoyment for your group. However, if your concern is feedback for the playtest then I would suggest skipping any alterations of the rules.
I don't think you would hurt anything. Guardian may be the only one that gives you trouble, and that is just because the monsters that barely hit will have to also go threw damage reduction.
LastHero is right, it is doubtful that you would hurt anything, but the researcher in me thinks this is a bad idea if you are filling out playtest surveys. For example, Think of Next as a new cancer killing drug and boredom as the cancer. Wotc is providing this drug to cure your cancer but they need you to tell them if it's working. Now you start taking your new drug and a couple of days in you have to take some asprin. After taking the asprin you develop a headache. Now you are faced with several possibilities, the drug you are taking causes headaches, the asprin caused the headache, the interaction between both caused a headache or none of the above you just got a headache. Now you could stop taking the asprin but then if you either did or did not get a headache after that you would have whats known as confirmation bias. Now you could both take and not take the asprin and see if the headache comes back with the asprin or without etc and rule in or out the asprin. However, you then return to the company and find that the drug didn't cure your cancer. Now you are faced with the skewed results of the asprin and can't tell weither the drug simply didn't work or it was the asprin interfearing with the drug.

Wow that got a lot more complicated than I intended. Anyway I myself wouldn't add anything that would inhibit the results of a pure and unbiased survey but if your not worrying about them then I say go wild
Two Weapon fighting is inferior most of the time so I would use the previous rules for that in particular. Otherwise its probably best to go with the new packet at least for one session and change it back if you don't like it.
First and foremost it's your game and if you wish to continue using these options then by all means do so. However, if your intent is to playtest and provide unbiased feedback on the options presented then perhaps this is not a good idea. It is almost guaranteed that adding any house rule or older options to the new packet will have consequences that may or may not be obvious during gameplay. These consequences could bias any interaction in either a positive or negative way. When presenting feedback on the options presented you may not be able to distinguish how adding these rules influenced your perception of the packet and could therefore give biased feedback based on how your rules interacted with the playtest. So really it's completely up to you and your group. If you wish to simply play the game and enjoy yourselves then be all means modify the game in any way you see fit to add fun and enjoyment for your group. However, if your concern is feedback for the playtest then I would suggest skipping any alterations of the rules.



So true, if you want to give the most accurate feedback.  At the very least state the changes you make in the survey. 
Thanks for the replies everyone. I decided against using the specialties from the previous packet and just used the new ones so we could give accurate feedback like Tsithlis suggested (nice analogy there btw). My archer wasn't too happy about it, but picked up the Ambusher instead and she seems okay with it. Will see how it actually plays out tomorrow. My players are hoping for the return of the more combat oriented specialties in future packets, mixed in with the more role-play and exploration ones from this newest iteration. One was even suggesting a module to allow for a combat specialty and a non-combat specialty for each character.