When viewing the Federal program, remember that even the smallest town gets almost 14% as a geographic differential, so the range from the least to the most is about 28%. That's quite conservative, since the difference between Bentonville AR or Big Spring TX and San Francisco is about 40%. Guess that's why the Postmaster still tends to be "rich" in a small town while starving in a major city.
Source of the lateral-transfer initiation is critical. If the company sends you somewhere, they assume responsibility for your balance of internal equity and external competitiveness. There is no reason for an employer not to pay a known competent performer at least as much as they would pay for a complete unknown recruit hired off the street in town. So if they choose to move you from Reno to fill an opening in Los Angeles, you should get a nice bump, even if part of it is tagged as a relocation stipend severable/adjustable or removable upon further relocation. Relocation pay can be treated just like international tax equalization, where you normalize to a base and adjust accordingly with the offset residuals clearly identified.
If you want to move from Little Rock to Manhattan, that's up to you. As you bid for internal openings elsewhere, you might not get fully equal treatment to the person management select as ideal for a "forced" relocation from SmallVille to BigApple. If you choose to transfer laterally from Palo Alto to Laramie or El Paso, you will be lucky if you don't take a steep cut in base, lose your bonus, or experience a pay freeze for many years. Normally, it's really difficult to laterally transfer folks from RichCity to CheapBurg, not only because of the differences in competitive norms but also because internal equity considerations frequently require public exposure of the accomodation terms you require the transferree to accept in order to make their pay palatably equitable among the local troops.