Well I don't have a lot of heli experience, but I'll tell you what I did when faced with the decision of whether or not to start upgrading my Blade 400.

I figured you're gonna need new servos including the tail rotor and a new gyro. Then new batteries as well as an ESC capable of handling the new stuff. Then aluminum CNC parts, then a new rotor blades, well you get the idea....

So I sold the Blade 400 after having only replaced the tail rotor servo with the digital version and bought a T-Rex 450SE V2. It comes with all the "bling" and I put the new JR DS287MG's on the swash with the DS3500D servo and G7703D gyro in a combo pack on the tail rotor.. While I may not have gone that high of a gyro on the Blade, I figured it would be available if I went to a 500 size heli. I'm not sure about using the DS290G as it's a bit lacking in the torque area, but Horizon does say it's the performance upgrade for the Blade 400. It would go right in the existing mount where the 3500 requires a different mount on the tailboom as it is a slightly bigger servo. Horizon calls the DS3500G "Perfect for 450- to 500-size helis".

Now all these upgrades are not cheap but I figure for about $100 to $150 extra, I now have a T-Rex 450 with all the trick stuf vs. a Blade 400 with all the trick stuff.

I'd suggest you evaluate where you want to go with the heli stuff as some here have spent all the money on Blade 400 upgrades and seem to be quite happy with them. I just chose to go another way.