Any musician can be replaced but the chances are that the band as a whole will be different whether in terms of talent, the personality of the replacement, how the group of people get along and work together among other things.
It's just like getting a dep in - it's never the same as gigging with your regular band mates. In time you will get used to the new member as they will get used to you but it will still never be the same. It's not necessarily better or worse, just different.
Having aspergers, I find it hard to get used to new people and fit in but i auditioned for a band in November. I've never felt such a connection to any group of people before, it was like I'd known them for years. 2 days later, they asked me to join their band and I couldn't be happier.
Their current bass player is leaving after 13 years with the band and I had a chance to meet him and the band last weekend to have a good look at the equipment that they use to find out if I needed to buy any new cables or gear of any sort. I heard them at sound check and they are really good but right away I knew that I was a completely different player to their current bass player so things will be different in that regard.
The spooky thing about the rehearsal was that I normally use my Warmoth/Status Jazz live. The photos I had seen of their current player was him using a 5 string. For some reason I took my black EBMM Sterling to the rehearsal instead and when I went to see them last weekend, their bass player was using a black EBMM Stingray.