Are there rules for increasing the strength bonus of a composite bow after it has been made, and calculating the cost of such modification? For example, if a character has a composite longbow +2 and increases her strength to 16, is it then possible to modify it into a composite longbow +3 or must she instead get a new bow crafted? This obviously becomes especially relevant as powerful characters gain access to magic weapons that do not exactly match their strength bonuses.
Probably not. The maximum strength bonus of the composite longbow is part of the weapon's construction. The description suggests that this is an inherent property when the weapon is built. While we have information about how this value is determined when the weapon is made, there do not appear to be mechanics for improving this property.
All composite bows are made with a particular Strength rating... A composite longbow can be made with a high strength rating to take advantage of an above-average Strength score; this feature allows you to add your Strength bonus to damage, up to the maximum bonus indicated for the bow. Each point of Strength bonus granted by the bow adds 100 gp to its cost.
It's like creating a weapon with the masterwork property. Whether a weapon is considered masterwork is determined when the weapon is created. The masterwork property is inherent, and cannot be added later.
The composite shortbow has nearly identical text, and should follow the same rules.
Requirement: This ability can only be placed on composite bows.
An adaptive bow responds to the strength of its wielder, acting as a bow with a strength rating equal to its wielder’s Strength bonus. The wielder can fire it with a lesser Strength bonus (and cause less damage) if desired.
This improvement costs a flat +1000gp and someone who can craft magical weapons or add new magic abilities. The upgrade is worthwhile; you only need to pay once, it does not raise the weapon's effective enhancement bonus, and the damage automatically adjusts if the wielder increases their Strength score later.