How would a fighter be changed from having static stability to having relaxed stability?
The first and most important step is to move the center of gravity back relative to the neutral point of the configuration. This is done either by shifting masses on the airframe back or by shifting the wings forward. This needs to be complemented by the addition of a multiple redundant and fast flight control computer.
Now the tail will add lift instead of downforce, so the wing area required for manoeuvring and take-off/landing can be reduced. Or performance improves - compare the minimum speed of the Mirage III to that of the Mirage 2000.
To see how much the wing area could be reduced, here is a comparison drawing of the regular SEPECAT Jaguar with one designed for the same requirements, but with relaxed stability (copied from this answer). The forward shift and reduced area of the wing should be obvious.

Comparison of conventional Jaguar (top half) and CCV version (bottom half) by Ray Whitford.
What is done to a specific airframe depends on the objectives for the move to relaxed stability: This could be much better supersonic range or better short field performance. For the first you will shrink the wing, for the second you better keep wing size but move it forward and heavy items back. The change from the one engine of the MiG-21 to two more powerful engines certainly helped to shift the center of gravity of the J-8 back.
Usually all of the methods you mention are used in combination (except for the clever aerodynamic device, unless you mean a sophisticated FCS):

The wings are resized/moved
The tailplane would be made smaller
Internal items will be shuffled around or exchanged to shift the center of gravity further back.