Playing music of different eras is difficult, in my opinion, as:

1) You have to pick up the style of that particular era
2) After that you have to present it in the style!!

For example, if you picked up Happy Birthday, would you be able to play it in these different styles, without changing the notes etc.??

On another note, what period of music are you more into? Romantic? Classical? Medieval? Renaissance? Baroque? 20th Century?

I really really enjoy Romantic particularly the works of Liszt!

Extremely difficult, I agree - something I haven't got used to, but faced for the first time in an audition with 3 different styles. It's fine to do any style if you can be consistent with it - even just working on one style for a day - but playing different styles in one performance takes an adjustment which I definitely haven't got the hang of.

Which periods I like? Bach (I'd love to get into Scarlatti, but haven't had time yet), and Romantic/strange 1900-1915 period: particularly Schumann at one end and Scriabin at the other. I like Haydn as well, but I can really take or leave Mozart and Beethoven. Not that I don't love hearing them played well, but given a limited time in which to work, I know what I want to concentrate on.

I'm not sure I could play Happy Birthday in the style of Bach. I'd have to improvise some insanely clever counterpoint to it, arrive at a pedal-point to build up tension, stick at it for a few bars, break away from it into a stretto - all beyond me, though I love playing that kind of thing once JS has written it down for me...