when in gnuradio top block is started can we change connect blocks?

[sp]

I have a question about gnuradio blocks when I define gnuradio blocks on a
top block and start top block can we changed connection between blocks when
tp is running with out stoping it.....
for  example i connect uhd to a null block, after some time i changed uhd
to another block?????
can any one guide me?
thanks in advance