Fix bug in XpressPersistent.update_var

[bknueven]

Fixes #3565

Summary/Motivation:

XpressPersistent.update_var was not behaving as I expected (see #3565)

Changes proposed in this PR:

• Add test demonstrating XpressPersistent.update_var does not use existing bounds when vartype changes #3565
• Switch the order of operations in XpressPersistent.update_var to change the variable type first, and then change the bounds

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[jsiirola]

Looks good, pending successful tests.