When attempting to use the replacement function of the /fill command to either place lava where an existing block had been or remove it to make room for said block fails in the snapshot. Not tested before 14w07a.

Examples of failed commands:
/fill ~2 ~ ~ ~8 ~ ~7 lava 0 replace lapis_block
Output States: [21:23:59] 0 blocks filled

/fill ~4 ~-2 ~ ~10 ~ ~7 lapis_block 0 replace lit_redstone_lamp
Output States: [21:27:49] 0 blocks filled

/execute @p[score_freeze=1] ~ ~-1 ~ fill ~-1 ~-1 ~-1 ~1 ~1 ~1 obsidian 0 replace lava (Attempt was to replace blocks of lava with obsidian when nearby.)
Output States: 0 blocks filled

A live example can be seen at http://www.youtube.com/watch?v=PCu7WSbK6eo&feature=youtu.be from 4:13 to 4:28.