TiVo 0 - Plays the TiVo guy boot movie
TiVo 1 - Goes to the Now Playing list. ( in 2.5, goes to the Season Pass Manager)
TiVo 2 - Goes to the To Do list. 
TiVo 3 - Goes to Wishlists (in 2.x+)
TiVo 4 - Goes to the "Browse By Name" screen. 
TiVo 5 - Goes to the "Browse By Channel" screen in 1.3, and to "LiveTV" in 2.0.  No, nobody knows why they changed it.  They changed it back to "Browse by Channel" for 2.5 though. 
TiVo 6 - Goes to the "Browse By Time" screen. 
TiVo 7 - Goes to the "Record Time/Channel" screen (manual record). 
TiVo 8 - Goes to the "TiVo's Suggestions" screen. 
TiVo 9 - Goes to the "Network Showcases" screen. 
TiVo TiVo - Goes to the Now Playing List (in 2.x+)
TiVo Slow (4.0 only) - Goes to "Messages and Setup"

This week I bought a Roamio after a great nine-year run with a couple of Series 2 boxes; really got my money's worth! 
Everything is wonderful except two frustrations:
1) The loss of the numeric shortcuts 5 & 6 on the remote control for Find by Time & Find by Channel.  Is there any other shortcut or backdoor way to get that functionality back?  I use them several times a day, and it's a real pain to have to dig through menus to get at them each time. ( Buttons 7, 8 & 9 apparently are currently unused.) 
2) Purely cosmetic, but I really miss the old TiVo GUI.  Is there any backdoor way of getting it back? 

Thanks for the reply, cwb3106! 
In my Roamio TCD846500, Settings > Displays only offers the following:
– Closed Captioning
– Lights on the Front of the DVR
– Customize Discovery Bar
– Video Window
...but nowhere in any of them do I see anything about changing the TiVo menus.  And I'm not seeing anything in the other Settings menus, either.  Am I missing something? 

After a week now under my belt with my new Roamio, I'm delighted to discover that complete functionality of Find by Time & Find by Channel are now combined in the Program Guide, and not only is it of course instantly accessible with a keypress, it's also actually easier for me to use than Find by Time & Find by Channel.  What did the trick was the addition of advancing forward or backward by one day with each press of the Advance or Replay keys, which wasn't available on my old Series2.  Oh, I'm one happy camper now! 

When on HD, hit Thumbs Down, Thumbs Up, Pause, Pause.  It'll switch to SD.  To get back to HD you'll have to get to settings though. 

For the hell of it, I tried it on my Roamio and it sounded a bong and went into a "One Moment..." screen, but returned right back to the HD menus.  So I guess the secret command is still in there, but the SD menus have really been removed from the Roamios. 

What model TiVos are there that have the SD option, but need a "secret code" to enable it?  As far as I know, if the SD menus are available, there's a front-facing setting in the menu to enable it. ( IOW, that code might make it faster to switch, but the menu setting remains.) 

I don't believe I've ever heard of any TiVo that still had the option for SD menus but that the menu setting to activate it was removed.