In my opinion if you change a vocalist with such a famous voice like Tony Hadley and Freddie Mercury for someone else you should also change your band name, not pretend that its the same band that had all the hits before, it isn't!
The vocalists more often than not make those songs hits, yes the band members may change every now and then such as the guitarists or drummers but the vocals make the songs. The only exceptions I can think of to this rule are that Bon Jovi lost a lot i their sound when Richie Sambora left, it was noticeable. The same can be said with Slash, the Guns N' Roses' long awaited album, "Chinese Democracy" still had Axl as vocalist but that "special" unique playing style of the man in the top hat was sorely missed.
I digress slightly so back to my point. If Spandau Ballet changed their name to Spandau or Second Ballet or Queen changed their name to Queen Two or something it would be ok. This would show that they were not trying to be the same band because it will sound different. When Fish left Marillion it wasn't the same sound when Steve Hogarth took over, although the music may still sound good, this was a different era and should be noted by a name change. Gary Kemp said "this is a new chapter for the band" but it wasn't, it was Spandau Ballet singing the same songs that only Big Tone could sing immaculately, with someone else, it was a rehash.
Bands change singers regularly before they become famous, that's normal, such as Simon Le Bon replacing Stephen 'Tintin' Duffy in Duran Duran. Adam Ant had to find a completely new band when Malcolm McLaren fiendishly stole the original Ants to form Bowwowwow but he kept the name and never looked back, even though the first album was decidedly dfferent from anything later.
Queen can never replace Freddie, or get someone else to sing the songs he made famous. He was a legendary frontman, a big act to follow in any terms but trying with a new singer and keeping the same name, singing the same songs, to me disrespects the legacy he left.
Sometines it works, when AC/DC's Bon Scott died in 1980, they almost quit but Brian Johnson took over on vocals and the rest is history. Bruce Dickinson replaced Paul Di'Anno after the first two albums for Iron Maiden and they had fabulous success. When Bruce left in the 90s to be replaced by Blaze Bayley the songs lacked his power up front until he returned.
So, it sometimes works, mostly it doesn't if you are a top, top band with a succession of hits. It is dangerous to still say, "this is still the same band" when the voice that pretty much sold the records is not there. It's a crazy point but imagine Blondie without Debbie Harry, would they still be Blondie?, no chance!