I can confirm that using this technique that data is not destroyed when merging partitions. I did that very thing today for a client who has a Tiger hard drive that originally contained two partitions. I booted from an external hard drive that contains OS X 10.5 and used Disk Utility to erase data from the extraneous Tiger partition. Afterward, I combined the two partitions into one. All the data on the main partition remained intact.

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