The changes between the Lot of Spirit and the Lot of Fortune are only for predictive techniques, one cannot use the the LoF as an alternative LoS or the other way round. The LoSpirit  has purely to do with the motivating impulse to act or do something whereas the LoFortune  has purely to do with those things that come back to us as a result of our actions.

 

However, the diurnal and nocturnal formulas are not very well understood and do cause the miscalculation of these two lots, in this respect the LoFort.  & the LoSpirit, can take each others place and cause a delineation mistake.

 the astrodictionary definition:
 
Pars Fortuna. The Part of Fortune. One of the Arabian Points (q.v.). A point that bears the same relation to the Rising Degree that the Moon bears to the Sun. It occupies the same House-position in a Figure based on a birth-moment, that the Moon tenants in a Solar Figure. Its symbol, a cross within a circle ED, is that utilized by astronomers to represent the Earth. It is the ancient Chinese symbol Tien, a field; used by the Egyptians to signify territory. It is generally considered that the House position of Fortuna is an indication of the department of life that will most readily contribute to the financial welfare of the native. If so it tends to lend emphasis to the theory that one succeeds best at that which one likes best; that congeniality of occupation is a contributing factor to success therein. v. Arabian Points.

 Robert Hand says about it:
 
"...the Part of Fortune is a primary indicator of prosperity, but it is more than that. It is created out of the longitudes of the Sun, Moon, and Ascendant, and therefore is composed out of the three of the most important places in the chart (only the Midheaven outside of these three is as important) and as a result becomes as important as they are. In modern astrology there is a tendency to regard the Part of Fortune as a minor point in the chart, but this was not the case with ancient astrology.  
 
In ancient astrology the Ascendant is the primary indicator of the physical body and health. The Moon is also a very important secondary indicator about the body. But there is a difference between them. The Ascendant was considered to be the body at its most physical, material level. The Moon was the living aspect of the body, in fact soul, but soul incarnated in a physical body. The Part of Fortune, being composed of both of these along with the Sun, is also an indicator of the living, physical body and its relationships with the physical and social world in which the native lives. Its capacity to signify prosperity comes from this. If the Part of Fortune is in good condition, the relationship between the native and the physical and social world in which he or she lives is one that supports the native and enables the native to live well. If the Part of Fortune is in poor condition, the native has a harder time living in relationship with the world that supports the native. The same can also be said about the relationships of the Part of Fortune and health."  
http://www.accessnewage.com/articles/astro/robhand2.htm

The Arabic Lots (Revisited)