Trappings and Powers
With the core rules as written, you choose your Trapping at the same time you choose your Power and there is no way of changing it after that point. If you wish to have a different Trapping for a Power you already have, then you need to get that Power again.
As far as I can see, there is nothing that explicitly states this to be the case in the core rulebook, although there are a number of points through the 'Trappings and Effects' and 'Powers' sections where it is implied. For this reason, you need to go to the official forums for clarification, and this post* confirms the above interpretation.
However, this only applies to the core rules. Individual settings and supplements may differ in their implementations. For example, several of the expansion books allow powers to be chosen on the fly (like Super Sorcery in Necessary Evil, which also allows you to choose a new trapping each time). This is also one area where it is common for GMs to house rule, and a search of the official forums will find plenty of suggestions for how to go about doing this.
Boost/Lower Trait is a different matter entirely. Both Attributes (Strength, Agility etc) and Skills (Fighting, Intimidation) count as Traits, and you choose at the point of casting which Trait you are going to target.
The first line of the Power description confirms the above interpretation:
This power allows a character to increase any of a target's Traits by one die type...
It is also worth noting that a lot of confusion with Savage Worlds rules comes from assumptions and 'filling in the gaps'. In this case, there is there is nothing in the Power description that limits its use to an individual Trait, or anything within the rules that links the Traits you can effect to the Trappings system. Confirmation of the above interpretation can be found in this thread* on the official forums.
Boost/Lower Trait however is one of the more common powers for settings to alter, as in its default form it is extremely flexible (this doesn't necessarily mean it is overpowered in this form though, just that individual settings tweak it to give a different feel). Some, such as Shaintar, split it into two individual Powers, one for raising and the other for lowering Traits. Other settings such as Hellfrost limit the Traits that the Power can be used on depending on the background and flavour of the Arcane Background.
*It should be noted that Clint is the official brand manager for Savage Worlds, and his answers on the subforum I have linked to count as official rules interpretations and are effectively RAW.