Converting Hex to decimal and back in TCL

expect1.10> set hex_to_decimal [expr 0x0000000A]
10
expect1.11> incr hex_to_decimal
11
expect1.12> set dec_to_hex [format %x $hex_to_decimal]
b
expect1.13>
expect1.16> set dec_to_hex [string toupper $dec_to_hex]
B
expect1.13>