Semantic Puns and Alternative Hanzi
According to 六書 
(http://en.wikipedia.org/wiki/Chinese_character_classification)

There are two kinds of this
1. 轉注：
Basically this mean the multiple characters may map in the same mapping if 
1) They are in same radical (category).
2) They sound similar 
Example: 老 (old) 考 (Originally meant old; deconstructed as 耂 derivative (老字头) of 老 lǎo "old" + 丂 qiǎo (phonetic). Later evolved to mean "deceased father". Modern meanings of to exam. (From http://en.wiktionary.org/wiki/%E8%80%83)


假借：
Basically this mean the a character is assigned to more than one senses.
Example: 蘇 (/su1/ 'revive') ＝ 更生('live again') → 甦
     云 (Originally Cloud, then become "say") →  雲 (Cloud)


Or you can search "通同字" or "通假字"