A tree cannot really be changed (per se) to anything other than what it is capable of supporting. Successfully grafting plants of different families is generally unlikely. The scion and rootstock must express some level of compatibility to successfully form a graft union.
This is to say that the grafting partners must have a sufficient level of anatomical and physiological similarities or they will not be able to form a bridge between their respective vascular tissues.
Guavas are in the Myrtaceae family and in theory may express some degree of graft compatibility with other plants in that family. However, before putting energy into grafting one variety of guava with another or a guava with another member of the myrtle family, you may want to develop a specific strategy.
Grafting is generally performed to confer some advantage for the resulting plant. Often a regionally appropriate rootstock, or one that has specific pest resistance is grafted to a scion that produces a desirable trait (fruit, flower, foliage etc.).
It would not be entirely unreasonable to graft a scion onto a rootstock just to experiment with novel forms of plants. However, since grafting is probably the most expensive form of propagation and often the least successful, I would suggest you study the matter carefully prior to investing in plant material and supplies.
The links below may provide you with a decent primer on basic grafting knowledge:
- Requirements for Successful Grafting: Compatibility
- Principles of Grafting and Budding
- Grafting Techniques in Guava (Psidium guajava)