Your sketch always starts at location 0 - the bootloader's actually at the end of memory - so you don't need to change anything.
can my program "overlap" with bootloaders's start address, so the chip will try to execute some part at the end of my program as a bootloader?
This article says that I can use full flash size for programs while having fuses configured for bootloader:http://arduino.cc/en/Hacking/Programmer
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