If you name ur son Jr, does ur name automatically change to Sr?

Yes, there cannot be a junior without a senior, and vice versa. For example, a man named Jack would not be called Jack Sr. until he had a son named Jack Jr. Otherwise there is an implication that makes no sense. Before Jack has a child, he is not considered senior to anyone. The same would hold true for Jack Smith I, Jack Smith II, and Jack Smith III.

Well, this is the theory anyway. In truth, parents can name their children as they see fit. Those children, when they become adults, may choose to follow a legal process for changing their names as they see fit.

Note: Just to clarify the way you worded your question, it is not all that common to actually name a son "Junior." Sometimes that is used as a nickname. The actual name would more often be something like Jack Smith, Jr.

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By the way, these are actually considered to be suffixes to the person's name.

See Wikipedia for full article on Generational Titles:

Junior, Senior, and Roman numerals

Generational suffixes are used to distinguish persons who would otherwise share the same name within a family. A generational suffix can be used informally (for disambiguation purposes, or as nicknames) and is often incorporated in legal documents.

The most common name suffixes are senior and junior, limited chiefly to American usage, which may be written with a capital first letter (Sr.) or in lower case (jr.) following the person’s name (with or without an interceding comma). The term 'junior' is correctly used only if a child is given exactly the same name as his or her parent.[1] When the suffixes are spelled out in full, they are always written with the first letter in lower case. Social name suffixes are far more frequently applied to men than to women. In French, the designations for a father and son with the same name are père ('father') and fils (‘son’). In Portuguese, common designations are Júnior (junior), Filho (son), Neto (grandson), and Sobrinho (nephew). In many other nations it is considered highly unusual or even inauspicious to give a son the same first name(s) as his father, removing the need for such suffixes.

Adding senior is only really done in comparison situations between the father and son. You don't write it on forms or anything.

Well, I guess you can if you want to be pretentious about the fact that you have a kid you named after yourself.

no